The East Bay Electric Lines were a unit of the Southern Pacific Railroad that operated electric interurban-type trains in the East Bay region of the San Francisco Bay Area. Beginning in 1862, the SP and its predecessors operated local steam-drawn ferry-train passenger service in the East Bay on an expanding system of lines, but in 1902 the Key System started a competing system of electric lines and ferries. The SP then drew up plans to expand and electrify its system of lines and this new service began in 1911. The trains served the cities of Berkeley, Albany, Emeryville, Oakland, Alameda, and San Leandro transporting commuters to and from the large Oakland Pier (the "mole") and SP Alameda Pier. A fleet of ferry boats ran between these piers and the docks of the Ferry Building on the San Francisco Embarcadero.

The East Bay Electric Lines became the Interurban Electric Railway (IER) in anticipation of the opening of the Bay Bridge Railway in January of 1939. This railway consisted of two tracks on the southern side of the lower deck of the San Francisco–Oakland Bay Bridge, running from the East Bay to the San Francisco Transbay Terminal. SP IER transbay commuter train service ended in July 1941.

Prior to the formation of the East Bay Electric Lines (and later Interurban Electric Railway), commuter services by the Southern Pacific Railroad and predecessor Central Pacific Railroad were run entirely by steam traction. The first railroad to operate suburban services in the East Bay was the San Francisco & Alameda Railroad, which was formed on March 25, 1863.

Through a series of mergers with the San Francisco, Alameda, & Stockton Railroad Company and the prior San Francisco & Alameda Railroad, the San Francisco, Oakland, & Alameda Railroad (SFO&A) was formed in June of 1871. The SFO&A would be absorbed by the Central Pacific Railroad in August. With the merger with the Central Pacific, trains would terminate at the Oakland Mole (a long ferry pier into the San Francisco Bay), starting in January of 1882. Suburban commuter services by the Central Pacific would be operated in the same manner after Southern Pacific took over.

In 1902 the San Francisco, Oakland, & San Jose Railway (SFO&SJ) would build a three-and-a-quarter-mile-long (5.2 km) pier from Emeryville into the San Francisco Bay. The "Key System Mole" as referred to by patrons would rival the Southern Pacific's Oakland Mole for speed and general service. The SFO&SJ interurban line was faster, quicker, cleaner, and quieter than the Southern Pacific's steam operations, which paled in comparison. Between 1902 and 1911, the appeal of the SFO&SJ, and later companies San Francisco, Oakland, & San Jose Consolidated Railway and Key System, would rival the Southern Pacific's steam operated commuter operations. After management changed hands in the Southern Pacific between Collis P. Huntington and Edward H. Harriman, the decline of revenue by the rivalry would force the Southern Pacific to electrify their lines in competition in 1911.

In 1911 Southern Pacific embarked on a task to double track and electrify its commuter lines. When the construction of catenary over the new lines was complete, Southern Pacific received a new fleet of 72-foot-long (22 m) steel interurbans from the American Car & Foundry Company in the later months of 1911. Electric service commenced on June 1, 1911 along the length of the Encinal Avenue Line to the Alameda Mole. For the first weeks, electric trains were relegated to midday service with steam trains taking over during rush-hours. When the electrification of the lines was completed, a passenger could board an East Bay Electric Lines interurban from either the Oakland or Alameda Moles, and travel to Dutton Avenue, Thousand Oaks, Albany, Berkeley, and Downtown Oakland. Long term plans called for extensions to Richmond and San Jose (to presumably link up with Southern Pacific's other interurban subsidiary, the Peninsular Railway), which never materialized. The company invested $10 million between 1909 and 1912 ($326 million in 2024 adjusted for inflation) upgrading the East Bay lines, though the increase in passengers failed to offset the expenditure.

In addition to interurban service, streetcar service began in 1912 through various sections of the cities it served. A series of smaller streetcars by the Pullman Car Company also served these lines until 1930. Between 1912 and 1930 there was little change to the services of the East Bay Electric Lines. Southern Pacific's efforts in the 1920s to relax work rules and increase fares failed. As a result, the company sought to merge East Bay Electric Lines with the rival Key System. Employee backlash halted these plans.

In 1930, all streetcar services ceased in Oakland and Berkeley as they had failed to turn a profit.^{[citation needed]} An internal report by Southern Pacific management in 1933 recommended total abandonment of East Bay electric services. Due to the widespread adoption of the automobile, the Great Depression, and high labor costs, the IER was rapidly losing both money and patronage, so a franchise was granted^[when?] to them for operation on the lower deck of the San Francisco Bay Bridge to the new Transbay Terminal, in order to entice new patrons. On November 14, 1934, the East Bay Electric Lines reorganized as the Interurban Electric Railway (IER).