Yeah, I guessed on that one. They may be able to use the third freight track that already passes through the station for that. The reason I said four is because there actually are just as many southbound commuter trains as northbound commuter trains at rush hour [1], since most of the trains stay overnight in Boston. The Amtrak trains are mostly southbound in the morning and northbound in the evening though.

It depends on what Amtrak's mathematicians decide, including whether electrifying the track is worth it.

[1] http://www.mbta.com/uploadedfiles/Documents/Schedules_and_Ma...