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EMD MP15AC
EMD MP15AC
EMD MP15AC
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EMD MP15AC
CP 1422. ex-SOO 1552, nee Milwaukee Road 486.
Type and origin
Power typeDiesel-electric
BuilderGeneral Motors Electro-Motive Division, General Motors Diesel, Canada
ModelMP15AC
Build dateAugust 1975 – August 1984
Total produced246
Specifications
Gauge4 ft 8+1⁄2 in (1,435 mm) standard gauge
Prime moverEMD 12-645E
Engine typeV12 two-stroke diesel
Displacement7,740 cu in (126.8 L)
Cylinders12
Cylinder size9+1⁄16 in × 10 in (230 mm × 254 mm)
bore x stroke
Performance figures
Power output1,500 hp (1,120 kW)
Career
NicknamesLittle Beaver
LocaleNorth America

The EMD MP15AC is a 1,500 hp (1,120 kW) diesel road switcher locomotive built by General Motors' Electro-Motive Division between August 1975 and August 1984. A variant of the EMD MP15DC with an AC/DC transmission, 246 examples were built, including 25 for export to Mexico, and four built in Canada.

Development

[edit]

The MP15DC's standard Blomberg B trucks were capable of transition and road speeds up to 60 mph (97 km/h), allowing use on road freights. Soon there was a demand for a model with an advanced AC drive system. The MP15AC replaced the MP15DC's DC generator with an alternator producing AC power which is converted to DC for the traction motors with a silicon rectifier. The MP15AC is 1.5 ft (457 mm) longer than an MP15DC, the extra space being needed for the rectifier equipment. The alternator-rectifier combination is more reliable than a generator, and this equipment became the standard for new diesel-electric locomotive designs.

The MP15AC is easily distinguished from the DC models. Instead of the front-mounted radiator intake and belt-driven fan used on all previous EMD switchers, these have intakes on the lower forward nose sides and electric fans. Side intakes allowed the unit to take in cooler air, and the electric fans improved a serious reliability issue found in its earlier DC sisters.[1][2]

Engine

[edit]

The MP15 used a roots-blown 12-cylinder 645E engine.[3] The engine is rated at 1,500 hp (1,120 kW).[4] The 645 series, introduced in 1966, was EMD's standard engine through the 1980s.[1] [5]

Original owners

[edit]

The six largest buyers, were all buying road locomotives with AR10 alternators throughout the 1970s, so the similarly equipped MP15AC was easily kept in good repair. 36 more units were sold to 8 other customers.[6]

Railroad Quantity Road numbers Notes
Great River Railroad 1 G-1
Kennecott Utah Copper Corporation 6 120–122, 701, 704, 906
Kodak Park Railroad 1 10
General Motors Electro-Motive Division 2 115–116
Missouri-Kansas-Texas 4 56–59
Milwaukee Road 64 434–497
Natl Harbours Board 4 8403–8406
Southern Pacific 58 2702–2759
Seaboard Coast Line Railroad 45 4000–4019, 4200–4224
Nacionales de México 25 9801–9825
Long Island 23 150–172
Louisville & Nashville 10 4225–4234
Terminal Alabama State Docks 1 803
DOE Showing Locomotive model 2 3727–3728
Total 246

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

EMD MP15AC

View on Grokipedia
from Grokipedia
The EMD MP15AC is a four-axle (B-B) diesel-electric switcher and light road-switcher locomotive manufactured by the Electro-Motive Division (EMD) of , featuring a 1,500 horsepower (1,120 kW) 12-cylinder 645E prime mover and (AC) transmission (AC/DC, with output rectified to DC for the traction motors) via an AR10 and D77 or D79 traction motors. Introduced in August 1975 as an upgraded variant of the earlier MP15DC model, the MP15AC was the first to incorporate EMD's Dash-2 series modular, solid-state AC electrical system, which improved reliability and maintenance compared to the direct current (DC) setup of its predecessor. Production continued until August 1984, with a total of 246 units built (including exports) for various North American railroads, including the , Southern Pacific, and Union Pacific, primarily for yard switching and short-haul freight duties to replace aging first-generation models like the GP7 and GP9. Key specifications include a top speed of 65 mph, starting of 62,000 pounds at 25% , continuous of 48,800 pounds at 9.3 mph, and an overall of 49 feet 2 inches, supported by Blomberg-B trucks with 40-inch wheels and a typical operating weight of 248,000 to 250,000 pounds. Fuel capacity varied from 1,100 to 1,600 gallons, enabling extended operations in rail yards or light road service, while features like Exane wiring and updated control systems enhanced its durability in demanding environments. Despite its technical advancements, the MP15AC faced market challenges due to its higher cost relative to older locomotives and the absence of crew comfort amenities like a , limiting sales primarily to railroads needing modern switchers for hump yard operations or terminal switching. Many units remain in service as of 2025, leased or owned by short-line and Class I railroads, underscoring the model's longevity and adaptability in the freight rail industry.

Development

Background and Design Goals

In the , the U.S. railroad industry grappled with escalating operational challenges, including the sharp rise in prices triggered by the 1973 oil embargo, which increased costs by approximately 2.5 times within a few years, and growing labor expenses as wages in the sector rose steadily since 1970. Electro-Motive Division (EMD), a leading locomotive manufacturer, sought to update its switcher lineup to address these pressures while meeting heightened demands for efficient yard operations amid expanding freight volumes and regulatory changes. The SW1500, introduced in 1966 as EMD's standard 1,500-horsepower switcher, had become outdated for high-intensity switching tasks, prompting the development of a more robust successor capable of handling both yard duties and light road service with reduced fuel consumption and manpower needs. The core design goals for the MP15AC focused on delivering 1,500 horsepower optimized for versatile performance in rail yards and short-haul routes, prioritizing reliability in high-cycle environments where locomotives endure thousands of starts, stops, and direction changes daily. This power level enabled efficient handling of switching assignments without excessive idling, directly countering cost pressures through features like expanded capacity—up to 1,600 gallons in the AC model—while supporting labor efficiency by minimizing the need for additional units or crew interventions in routine operations. EMD aimed to create a "road-switcher" that bridged traditional yard work with emerging needs for localized freight movement, ensuring durability under intensive use to lower long-term maintenance and operational expenses. Building on the MP15DC as its foundational predecessor, the MP15AC's initial development commenced in , with the first production order placed in and deliveries beginning in of that year. This AC variant marked EMD's strategic pivot toward alternator-based transmission systems, which offered superior traction control and electrical efficiency over direct-current generators, particularly beneficial for the precise low-speed maneuvering required in congested yards. The design extended the frame by about 1.5 feet to accommodate the equipment, reflecting a deliberate to enhance starting and overall adaptability in response to industry shifts.

Engineering Innovations

The EMD MP15AC introduced an transmission system that marked a significant advancement over the DC generator-equipped MP15DC, utilizing an to generate , which is then converted to DC via a silicon rectifier for the traction motors. This design provided superior low-speed traction performance, enabling higher starting for yard switching tasks, while also reducing maintenance requirements through greater and fewer wearing parts compared to traditional DC generators. To address cooling challenges in dusty yard environments, the MP15AC featured relocated side air intakes at the lower hood level near the pilot end, drawing in cooler ambient air rather than hot engine bay air, combined with electric-driven fans for more reliable operation. Borrowed from EMD's "Tunnel Motor" locomotives like the SD40T-2, these upgrades minimized overheating and ingestion issues that plagued earlier DC models, enhancing overall durability in harsh switching conditions. The locomotive's cab design incorporated enhancements for better operator during extended switching duties. The MP15AC featured an overall length extended by up to 18 inches compared to earlier MP15DC models to accommodate the rectifier equipment. As a dedicated end-cab switcher, it offered excellent forward and rearward visibility through large windows, facilitating precise maneuvering in congested yards without the need for frequent repositioning. Due to its compact frame delivering robust 1,500 hp in a versatile package suited for intensive yard work, the MP15AC earned the affectionate nickname "Little Beaver," evoking the industrious nature of a in tirelessly building and maintaining rail operations.

Technical Specifications

Prime Mover and Power Output

The EMD MP15AC utilizes the 12-645E prime mover, a V12 developed by (EMD) as an evolution of the earlier series. This engine employs supercharging for uniflow scavenging, enabling efficient operation in low-speed, high-torque applications typical of yard switching. With a bore of 9 1/16 inches (230 mm) and stroke of 10 inches (254 mm), each cylinder displaces 645 cubic inches, yielding a total of 7,740 cubic inches (126.8 L). The 12-645E produces a continuous power output of 1,500 horsepower (1,120 kW) at 800 rpm, optimized to deliver robust low-end for precise control during shunting maneuvers without excessive speed. This rating supports the locomotive's multi-purpose role, balancing and performance in confined yard environments. The fuel system features a total capacity of 1,100 to 1,600 gallons, typically divided into dual side-mounted tanks to facilitate prolonged shifts, with idle consumption rates around 3.8 gallons per hour for similar 12-645E installations. Maintenance is enhanced by the engine's modular unitized power assembly design, where cylinder liners, pistons, and heads can be replaced as integrated units, simplifying field overhauls and mitigating common two-stroke issues like wear and scavenging inefficiencies. This approach, introduced with the 645 series, improves reliability and reduces downtime compared to earlier non-unitized designs.

Transmission and Electrical Systems

The EMD MP15AC employs an AC electrical transmission system, where the prime mover drives an that generates three-phase (AC), which is subsequently converted to (DC) via a silicon diode rectifier for powering the traction motors. This setup, utilizing an AR10 , provides smoother power delivery and precise control compared to traditional DC generators, facilitating efficient low-speed operations in yard and switching duties. The locomotive is equipped with four D77 or D79 DC traction motors, one mounted on each within the B-B truck configuration, enabling direct drive to the wheels without cardan shafts. These motors deliver a starting of 62,000 pounds-force, with continuous rated at 48,800 pounds-force at 9.3 mph, supporting the unit's role in heavy switching tasks. Auxiliary electrical systems include an onboard auxiliary generator for powering lights, control circuits, and other non-traction needs, along with mechanical blowers for cooling. The MP15AC does not feature as standard equipment, relying instead on air brakes for stopping, which suits its primary yard service application. The alternator-rectifier design enhances overall reliability by minimizing arcing, brush wear, and maintenance demands associated with DC generators, contributing to longer service intervals in demanding environments.

Physical Dimensions and Performance

The EMD MP15AC measures 49 feet 2 inches (14.98 m) in length over the coupler pulling faces, 10 feet 3 inches (3.13 m) in width over the grab irons, and 15 feet (4.57 m) in height above the rails. Its total loaded weight on the rails averages 248,000 to 250,000 pounds (112,500 to 113,400 kg), providing stability for yard and light road switching duties. These dimensions allowed the MP15AC to navigate tight yard clearances while maintaining a compact footprint suitable for industrial and operations. The employs a B-B configuration with four on standard gauge tracks measuring 4 feet 8½ inches (1,435 mm), equipped with 40-inch (1,016 mm) diameter wheels. This setup distributes weight evenly across the axles, yielding an approximate loading of 62,000 pounds (28,100 kg) to minimize track stress in low-speed service environments. The design also incorporates a capacity of 28 cubic feet, enabling effective application of to the rails for enhanced traction during starts and stops on uneven or contaminated yard tracks. In terms of performance, the MP15AC achieves a top speed of 65 mph (105 km/h), sufficient for both yard maneuvering and short-haul road assignments. It delivers a continuous of 48,800 lbf (217 kN) at 9.3 mph (15 km/h), supporting reliable pulling of moderate consists in switching scenarios, while the starting tractive effort reaches 62,000 lbf (276 kN) for overcoming initial resistance. These capabilities, influenced by the 1,500 mover, underscored the MP15AC's versatility in demanding service conditions without requiring extensive modifications.

Production

Manufacturing Details

The EMD MP15AC was manufactured from August 1975 to August 1984, spanning nearly a decade of production at Electro-Motive Division facilities. Output peaked during 1976–1978, when annual builds reached 32 to 35 units, reflecting strong demand for this road switcher design amid railroad modernization efforts. Primary assembly occurred at EMD's main plant in LaGrange, Illinois, where the vast majority of the 246 units were constructed. An exception involved four units built by General Motors Diesel Division (GMDD) at its facility in 1984, destined for export to the National Harbours Board in . Over the production run, build variations emerged to refine and . Early units incorporated minor cab adjustments for improved visibility and operator comfort, while later examples featured subtle dimensional changes to accommodate component updates. These evolutions addressed operational feedback without altering core specifications. at EMD emphasized rigorous pre-shipment testing to ensure reliability. Each underwent evaluations simulating extended operation, typically exceeding 100 hours, to validate , electrical systems, and overall under load—protocols demonstrated in contemporary EMD testing for similar models like the GP40-2.

Total Output and Builders

A total of 246 EMD MP15AC locomotives were produced, consisting of 217 units for operators, 25 for export to , and 4 built for Canadian service. Of these, 242 were constructed by the Electro-Motive Division (EMD) at its facility in LaGrange, Illinois, while the remaining 4 were built by (GMD) in . The EMD units encompass the U.S. and Mexican batches, with serial numbers ranging from 756025-1 to 836053-1, interrupted by gaps for production of other models. The export order of 25 MP15AC locomotives represented the primary non-U.S. batch for Mexico's (NdeM), delivered as road numbers 9801–9825 with serial numbers 818040-1 through 818040-25 in 1983. These units were tailored for yard and short-haul service on the NdeM network, marking a significant international application of the MP15AC design beyond North American borders. The GMD-built locomotives, assigned serial numbers A4423 through A4426, were produced specifically for the National Harbours Board in as numbers 8403–8406.

Operators and Service

Original Purchasers

The EMD MP15AC was initially purchased by a variety of North American railroads and industrial operators seeking versatile switchers capable of handling both yard duties and light road service, particularly as aging first-generation locomotives like GP7s and GP9s required replacement during the mid-1970s railroad mergers and modernization efforts. A total of 246 units were produced between 1975 and 1984, with the majority acquired by Class I railroads for switching operations. The largest U.S. purchaser was the Milwaukee Road, which ordered 64 MP15ACs between December 1975 and March 1976 for yard service across its network, including operations at Southtown Yard in the Twin Cities and Portage, Wisconsin. Southern Pacific followed with 58 units delivered in 1975, primarily for hump yard switching, such as at Eugene Yard in Oregon where they were often paired with yard slugs. The Seaboard Coast Line acquired 45 units from October 1977 to February 1978, deploying them at coastal terminals including the Hooker Point Seaport in Tampa, Florida, and North Charleston, South Carolina. Other notable U.S. buyers included the , which purchased 23 units in March-April 1977 for passenger yard operations and occasional commuter train assistance, such as rescue moves and switching at Fresh Pond Yard. The Louisville & Nashville acquired 10 units in March 1978 for yard duties. Industrial operator obtained 6 units between December 1976 and 1978 for mining rail service at the , hauling ore trains to mills. Additional purchasers included the Missouri-Kansas-Texas (4 units, October 1980) and various government and industrial users. Internationally, Nacionales de México bought 25 units in July-September 1982 for yard switching near U.S. border crossings, supporting cross-border freight operations.

Subsequent Use and Rebuilds

Following the Road's in 1985, 32 of its 64 MP15AC locomotives (originally numbered 434-465) were transferred to the , which acquired significant portions of the Milwaukee's assets during the reorganization process. These units were renumbered as Soo Line 1500-1531 and continued in yard and transfer service, with 32 eventually leased or purchased by Union Pacific in the late and early , becoming UPY 1397-1428 for similar duties across the UP system. The 1996 merger between Southern Pacific Transportation Company and Union Pacific resulted in the transfer of 57 of SP's 58 MP15AC units (originally 2702-2759) to UP, where they were renumbered UPY 1431-1487 and employed primarily in hump yard switching and local freight operations at locations such as . A subset of these, including former SP 2756 (UPY 1484), were later reassigned to shortline operations after retirement from UP in 2016, sold to entities like S&S Sales and Leasing for continued industrial service. Rebuild programs in the and extending into the focused on modernizing MP15AC units to meet emissions standards and prolong operational life beyond 40 years, often incorporating controls and updated electrical systems. VMV Enterprises in , conducted several such overhauls, including remanufacturing 12-645E engines and enhanced cooling systems. By 2003, VMV had remanufactured additional examples with new wiring, traction motors, and alignment-control draft gears to address wear from heavy switching, enabling resale to leasing companies for secondary markets. Dozens of MP15AC locomotives entered shortline and industrial service by the 1990s, including exports of used units to and for lighter-duty applications on regional carriers and operations. Representative examples include former UP and SP units sold to Canadian shortlines and Mexican operators such as Transportacion Ferroviaria Mexicana, where they handled transfer runs and plant switching with their reliable 1,500 hp output. These secondary roles leveraged the model's Blomberg trucks and AC transmission for versatile performance on uneven track. Most MP15AC units remained active into the on Class I and shortline railroads, but retirements accelerated due to the locomotive's age, with challenges including parts scarcity for obsolete components like the AR10 alternator and D78 traction motors. By 2016, Union Pacific had retired several ex-SP and ex-Milwaukee units, some of which were scrapped for their prime movers while others were parted out or sold intact to preserve operational fleets amid rising maintenance costs. As of 2025, dozens remain in service on shortlines such as the New York & Atlantic Railway and in lease fleets, underscoring ongoing adaptability despite increasing retirements.

Preservation and Legacy

Surviving Examples

Several MP15AC locomotives have been preserved, primarily in museum collections, highlighting their role in yard and industrial service. One prominent example is , constructed in April 1981 (EMD 806035-1), which served in the extensive intra-plant rail network at in . This unit was donated to the Rochester & Genesee Valley Railroad Museum in Industry, New York, in 2019, where it is maintained as a static display, preserving the model's AC transmission features for educational purposes. Restoration efforts for preserved MP15ACs often rely on volunteer labor at nonprofit museums, with a focus on refurbishing the original alternator-rectifier systems and 12-645E prime mover to ensure historical accuracy. These projects emphasize retaining the locomotive's 1,500-horsepower configuration while addressing wear from decades of service. Few MP15ACs are preserved in museums, as most of the 246 built remain in or stored on shortlines and Class I railroads, underscoring the model's longevity. Key challenges in these restorations include procuring components for the aging 645-series engine, as has prioritized production and support for the newer 710-series power plants. However, aftermarket suppliers provide remanufactured and exchange parts, such as pistons, bearings, and turbochargers, enabling ongoing preservation work despite the shift in focus.

Operational Impact

The introduction of the EMD MP15AC in marked a significant step in the adoption of (AC) electrical systems within the switching locomotive segment, as it was the first EMD switcher to incorporate an producing , which was then rectified to DC for the traction motors. This innovation, part of EMD's Dash 2 modular electrical system, improved power transmission efficiency and reliability over previous DC generators, paving the way for broader integration of AC electrical systems in subsequent EMD models such as the MP15T (built 1984-1987), which featured enhanced fuel economy through a turbocharged engine. While initial adoption was tempered by higher costs compared to the DC variant—resulting in 246 units produced—the MP15AC's design demonstrated the viability of AC electrical systems for yard and road-switching duties, influencing the evolution toward more versatile multi-purpose locomotives in the rail industry. In the context of the 1980s rail deregulation under the Staggers Act, the MP15AC contributed to operational efficiencies in rail yards by enabling more reliable and versatile switching operations, often replacing older GP7 and GP9 units with its 1,500 horsepower output, Blomberg-M trucks for better adhesion, and a 1,600-gallon fuel capacity suitable for extended light road service. This alignment with deregulation-driven cost reductions allowed railroads to streamline yard crews and boost productivity in freight handling, as the locomotive's solid-state controls and AC system minimized maintenance downtime in high-cycle environments. Its role in these efficiency gains is evidenced by sustained use on major carriers, with fleets like Union Pacific repainting and integrating dozens of units from mergers into ongoing service. The MP15AC's legacy is underscored by its enduring reliability and strong presence in secondary markets, with many of the 246 built units still operational on Class I and shortline railroads as of 2025, reflecting a service life that outlasted many contemporaries due to the robust AC electrical setup. High resale value in the used market is highlighted by examples such as the Grafton & Upton Railroad's successful on leased MP15ACs, which performed reliably in daily operations and supported fleet consolidation efforts. Culturally, the MP15AC has become a symbol of the 1970s-1980s transition in American railroading, representing the shift from traditional end-cab switchers to more road-capable designs amid industry modernization; it frequently appears in photography of carriers like the and CSX, and its distinctive hooded front and radiator intakes have inspired popular HO-scale models, fostering appreciation among enthusiasts for its historical role in yard operations.

References

  1. https://www.gatx.com/products/mp15ac/
  2. https://www.thedieselshop.us/Data%20EMD%20MP15.HTML
  3. https://www.trains.com/mrr/news-reviews/reviews/staff-reviews/athearn-ho-mp15ac-diesel-switcher/
  4. https://www.american-rails.com/mp15dc.html
  5. https://utahrails.net/loconotes/MP15.php
  6. https://rosap.ntl.bts.gov/view/dot/10319/dot_10319_DS1.pdf
  7. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=2000L3SE.TXT
  8. https://live.dovetailgames.com/live/train-sim-world/articles/article/a-multi-purpose-marvel
  9. https://www.american-rails.com/645.html
  10. https://projectrail.wordpress.com/2008/11/12/prime-mover-emd-645-data/
  11. https://horizonrail.com/fuel-consumption/
  12. https://www.progressrail.com/en/Segments/Locomotive/Engines/Engine_Product_Support.html
  13. https://trainweb.us/emdloco/mp15.htm
  14. https://www.trains.com/trn/train-basics/ask-trains/ask-trains-from-august-2005/
  15. https://www.facebook.com/groups/775685475813757/posts/1384695944912704/
  16. https://www.youtube.com/watch?v=NvF38yHp2W0
  17. https://www.pinterest.com/pin/242842604878949090/
  18. http://www.trainsarefun.com/lirr/lirrMP15.htm
  19. https://utahrails.net/bingham/kcc-utah-locos.php
  20. https://www.sec.gov/Archives/edgar/data/1055291/000095013406007229/c03926e10vk.htm
  21. https://utahrails.net/up-diesel-roster/upy-03.php
  22. https://utahrails.net/utahrails/slgw-locos-diesels.php
  23. https://eastcoastlocomotive.com/locomotive-sales-and-leasing/
  24. https://www.flickr.com/photos/rgvrrm/46066400851
  25. https://www.railpictures.net/photo/721929/
  26. https://www.epowerrail.com/Products/CategoryEngine-EMD/SubMajor-Component
  27. https://www.springcreekmodeltrains.com/product-category/ho-scale/athearn-ho/athearn-ho-athearn-genesis-ho/emd-diesel-locomotives-athearn-genesis-ho/mp15ac-diesel-switcher-emd-diesel-locomotives-athearn-genesis-ho/
  28. https://www.trains.com/trn/news-reviews/news-wire/23-grafton-upton-standardizes-locomotive-roster-with-mp15acs/
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