Washington was served by the following major railroads at Union Station. 

Railroads that Served Union Station in 1950

 

Name: Atlantic Coast Line

Commonly Used Abbreviation: ACL

For more information: 

http://www.aclsal.org/

The ACL served Washington, D.C. and New York via interline agreement over the RF&P and PRR. The railroad served much of the Southeast United States including Florida. 

Name: Baltimore & Ohio Railroad

Commonly Used Abbreviation: B&O

For more information:

http://www.borhs.org/

Served 13 states from the mid-Atlantic to Chicago.

 Name: Chesapeake & Ohio Railway

Commonly Used Abbreviation: C&O

For more information:

http://www.cohs.org/ 

Served Washington, D.C. via trackage rights over the Souther Railway from Orange, Virginia. A major coal haul and heavy industry railroad from the industrial heartland of Ohio, Michigan, West Virginia, and Kentucky to the eastern seaboard and Chicago.

Name: Pennsylvania Railroad

Commonly Used Abbreviation: PRR

For more information:

http://www.prrths.com/

The largest of the major railroads serving Washington, D.C. with service to New York, Chicago, St. Louis, Pittsburgh, Detroit and many other cities. Calling itself "the standard railroad of the world" the PRR once carried one of every 10 passengers in the United States.

Name: Pullman Company

For more information: 

http://www.newberry.org/sites/default/files/researchguide-attachments/PullmanGuide.pdf

Contractor to the railroads provided and operated most of the sleeping cars operated by major railroads.

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 Name: Railway Express Agency

Commonly Used Abbreviation: REA

For more information: 

From Trains Magazine

Fast intercity package express and shipping company provided by rail. Similar in concept to service provided by today Federal Express or United Parcel Service.

Name: Richmond Fredericksburg & Potomac

Commonly Used Abbreviation: RF&P

For more information:

http://www.rfandp.org

A bridge line between Washington, D.C. and Richmond Virginia.  Major business was through-traffic.

Name: Seaboard Airline

Commonly Used Abbreviation: SAL

For more information: 

http://www.aclsal.org/

Served Washington, D.C. and New York via trackage rights over the RF&P and PRR.  Railroad served much of the Southeast United States including Florida

Name: Southern Railway

Commonly Used Abbreviation: SRY

For more information:

http://www.srha.net/ 

Served from Washington, D.C. to much of the Southeast including Atlanta, New Orleans, Jacksonville, and Cincinnati.

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Name: Washington Terminal Company

Commonly Used Abbreviation: WTCo.

For more information: 

http://en.wikipedia.org/wiki/Washington_Terminal_Company

Washington Terminal Company was the railroad company formed and jointly owned by the B&O and PRR railroads to build and operate Union Station and associated infrastructure, to include Ivy City Shops.

Other notable railroads that served Washington, D.C. in the 20th Century include:

  • Chesapeake Beach Railway
  • Washington Baltimore & Annapolis Railway
  • East Washington Railway
  • Washington & Old Dominion Railway
  • Capitol Transit Company
  • Capitol Traction Company

A fairly complete list of the railroads that serve or have served Washingto, D.C (including predecessor lines) is available on wikipedia.