Some Metro riders heading to Maryland on Monday got an unexpected trip to Virginia instead when a Green Line train bound for Branch Avenue ended up at the Pentagon station.

The train then offloaded there, leaving commuters to make their way back to the other side of the Potomac River. Riders reported confusion amid the delays -- an increasingly frequent complaint as Metro line colors do not necessarily correlate with the tracks that trains use, whether intentionally or not.

In this latest case, the misdirected train was due to the Green Line train overrunning a signal at L'Enfant Plaza about 4:30 p.m., according to Metro.

"In an effort to minimize the impact to customers, the Green Line train was directed to offload passengers at Pentagon station, where the station manager directed them to a Yellow Line train heading to L'Enfant Plaza," said Metro spokesman Philip Stewart.

The incident is under investigation.

For riders, it was a headache, as those unfamiliar with the Pentagon station had to change floors to board an inbound train back to L'Enfant Plaza. But it also illustrated a source of confusion that is becoming more common when the agency intentionally changes routes.

Metro has been changing its service patterns from its long-standing color-matched scheme -- and it will continue to make more changes in coming years with the opening of the Silver Line.

In June, Metro started its Rush Plus service plan, which runs some Blue Line trains on the Yellow Line's tracks. And trains originating at Green Line stations could end up at Yellow or Blue ones -- intentionally -- while some Blue ones could end up at Orange ones.

Then add to the mix occasional reroutes due to track work.

Confused yet?

The current Rush Plus service plan isn't going to stick around long. When the Silver Line begins service in what is now planned to be late 2013, Metro will need to make room for those trains. Currently, the agency plans to have Silver Line trains run all the way to Largo Town Center on what is now the eastern end of the Blue Line. That will cause other trains to shift.