The Seaboard Air Line Railroad's Miami Subdivision was a railroad line in Florida connecting the network to the city of Miami. Officially beginning in Wildwood on the company's Main Line, the Miami Subdivision split from the Main Line five miles south of Wildwood in Coleman and ran through rural Central Florida before reaching South Florida and Miami, a distance of 273 miles. The Miami Subdivision was built during the Florida land boom of the 1920s and a vast majority of the line is still in service today. It is now CSX Transportation's Auburndale Subdivision from Auburndale to Mangonia Park and the state-owned South Florida Rail Corridor south of there.

The Miami Subdivision began in Wildwood on the Seaboard Air Line's Main Line, where they had a passenger depot and a large classification yard. From Wildwood, the Miami Subdivision ran down the Main Line five miles to Coleman (which was double-tracked between these two locations). In Coleman, the Miami Subdivision split from the Main Line and turned south-southeast, while the Main Line continued southwest to Tampa. From Coleman, the Miami Subdivision passed through Center Hill and continued south through wetlands of north Central Florida down to Polk City and Auburndale. In Auburndale, it crossed the Main Line of the Atlantic Coast Line Railroad, one of the Seaboard's competitors.

From Auburndale, the line continued south through Winter Haven and West Lake Wales. At West Lake Wales, it crossed the Seaboard Air Line's Valrico Subdivision, which provided a route west toward Tampa. Further south, it continued through southern Central Florida, passing through Avon Park and Sebring where closely paralleled the rival Atlantic Coast Line Railroad's Haines City Branch.

After Sebring, the line turned east and southeast to Okeechobee. From here, it continued southeast near the northern edge of Lake Okeechobee in a nearly straight line, passing through Indiantown before turning south as it entered West Palm Beach.

From West Palm Beach, the line continued south along a route largely paralleling the Florida East Coast Railway (FEC), which existed a few miles closer to the coast. The Seaboard line ran just to the west of the central areas of Boynton, Delray, Boca Raton, Deerfield, Fort Lauderdale, and Hollywood.

At Port Everglades Junction just south of Fort Lauderdale, the Port Everglades Belt Line Railroad connected the Seaboard line with Port Everglades, which opened in 1928. The Port Everglades Belt Line ran east to the port along the current route of Interstate 595.

South of Hollywood, the line turned southwest through Opa-locka before turning back south through Hialeah. In Hialeah, the line crossed the Florida East Coast Railway's Little River Branch, a junction which is known today as Iris Interlocking. A freight yard and maintenance shops were also located in Hialeah, along with a connection to the company's Homestead Subdivision.

South of Hialeah, the line turned southeast along the Miami Canal toward downtown Miami. A passenger depot serving Miami was built at 2206 NW 7th Ave in the Allapattah neighborhood. Track would continue beyond the passenger depot south and east into downtown. It crossed the FEC's main line near NW 11th Street and connected with the Miami Municipal Railway, which continued the line a short distance to the city docks at Miami's original port (located at the site of Museum Park).