A maglev line between Washington and Baltimore makes almost as much sense as flying between Washington and Baltimore.
The intent seems to be, as I understand it, to use a freely-provided WAS-BAL line as a loss leader to sell the US on a WAS-NYP (and, apparently, possibly WAS-BOS) line. Like a lot of project first phases (CAHSR, I'm looking at you), the initial phase really only makes sense as part of a bigger project.
Edit: Though I can't help but wonder what the effects of being able to pop between downtown DC and downtown Baltimore in something like 20 minutes would be. Considering that the IOS would need to be priced to fill up a decent number of seats on just a short section...