Now Google Maps can tell you. They cover all MTA modes including bus, light rail, Metro subway, and MARC commuter rail trains. And they explain how it works in this video.
Since Google collected schedules as well as routes, they can also tell you how long your public transit trip will be at specific times of day.
Some Baltimore-related examples here, like how to get from the Security Square Mall to 500 East Joppa Road in Towson at 7:00 PM. It also works for trips between Baltimore and DC.
Via Megan McArdle.