Companion map to the DC RER thing. The Metro network implied here is slightly different to what’s shown in that other Baltimore Metro map I did so very long ago; Line F is no longer a diesel shuttle from Cockeysville to Monkton—that was taken over by a northern RER branch—but is instead a Metro line that uses the alignment they should have used for the light rail (goes through Towson instead of around it, f’rinstance); and Line E generally instead of exactly follows the alignment of the late and very much lamented Red Line (and has a branch to Edgemere using an old freight line). The B/C/D light rail line is still there, of course, functioning as a local line for the RER L north-south mainline.
In an earlier draft, there were eight southern branches peeling off the RER L from the south. This was troubling, because the max number of branches you can have off a two-track commuter rail mainline and still have some semblance of frequent service is four. (Even the RER C in Paris, in all its impenetrable complexity, only has three branches from the north/west and four from the south/east.) So I broke some of the minor Baltimore City branches off into their own line (RER N), which I had terminate at the stub-end platforms at Camden Yards,* and turned the Fort Meade branch into a shuttle.
I didn’t do a “realistic” version of this map. If I did, the RER K would just run from Odenton to Edgewood, and the RER L would run from Laurel to Camden Yards.
(Thanks also to twitter user Transit Plans for suggesting a sorely needed transfer station between the two big lines.)
*I wanted the RER N to terminate at North Avenue, but the big chokepoint here is the Howard Street Tunnel. From all the photographs and aerial views I’ve seen, it looks like it only has one track, and given space restrictions in the area, any hypothetical replacement would probably only have two at most. I needed those two tracks for the RER L north-south mainline. Two tracks can take four branches, so I was uncomfortable having seven branches clog up that section of track. Furthermore, one track can only take two commuter rail branches, so if we allow the RER L one track under Howard Street, half the trains coming from the south would have to terminate at Camden Yards anyway.