FDW
Banned
I don't see a subway connection between DC and Baltimore. However, the reason isn't so much a lack of investment...it's more that you're likely to see one of the railroads (B&O, Pennsy, etc.) up frequencies and/or improve the alignments on the existing services.
I would think otherwise.
However, this does raise an interesting thought: Could you get a development corridor paralleling the main rail lines running from Baltimore to DC? Also, it does seem plausible that you could eventually get an integrated alignment (look at the Metro running right next to the RF&P main line these days), but I think that project might get shelved.
There already is steady corridor of development between the two cities IOTL, so yeah, I assume the same case ITTL.
Another bit of food for thought: Could more Metro lines be oriented towards Washington Union Station? IRL, only the Red Line runs through there...would it be possible to slide one or two of the other lines there and have Union Station displace one of the other main junctions (preferably including the Dulles-serving line) so as to make transfers easier?
Well, given that DC will have more metro lines than OTL in my plan, there will most definitely be more than one serving the station.