I'm not sure about the schedule for the MARC trains but do make sure that they are running after the show. Seems that was the issue at one time for evening shows in Baltimore. Of course, I don't know the times of this concert either.
You could even consider checking out the Light Rail or the Subway lines and spend the afternoon in some place not far from one of their Park And Rides. Maybe lunch and browse around in either Hampden, Towson, Hunt Valley or some other area of interest. (Not that there are lots of them alone these very limited lines.) Hampden might be fun for lunch and Hon watching. There's also a record store somewhere in Hampden that a few people around here said they liked. Then go over to one of the Park and Rides where, unlike super-gouging DC, it is FREE to park your car. I'm sure the schedules and area maps are online.
Or even just use one of the out of the city Park And Rides to park, take it downtown and hang around, then go off to the show. Again, do check the schedules. Between the two, I'd try to stick with the Light Rail over the Subway as the closer to the city areas are safer, both for hanging out in and leaving your car.