Today is the day some Metro subway commuters have to start paying a bit more attention to which train they ride or else they are likely to end up miles from their planned destination.

For the first time, trains sharing the same track will be headed for different points. The key is to remember that Orange Line trains run between the new Ballston station in Arlington and New Carrollton in Prince George's County and Blue Line trains will run between the Stadium-Armory station in the District of Columbia and National Airport. They will share the track between Rosslyn and Stadium-Armory.

Riders wishing to go to any stations between Stadium-Armory and Rosslyn can take either an Orange or Blue line train.

Riders wanting to go to Minnesota Avenue, Deanwood, Cheverly, Landover and New Carrollton must ride a train marked "New Carrollon."

Anyone wanting to go to Arlington Cemetery, Pentagon, Pentagon City, Crystal City or National Airport should get on a train marked "National Airport."

Those wishing to go to the Arlington Courthouse, Clarendon, Virginia Square or Ballston stations -- the four that opened Saturday -- should ride a train marked "Ballston."

Blue Line trains have signs with blue backgrounds and Orange Line trains have markers with orange backgrounds.