Map:
https://goo.gl/maps/oGkVLXvXyVkgiT9U6
About 4 or so years ago I worked in the old Federal Building on High St, just up the hill, and lived on Asylum Hill. Used to hang out at the Federal Cafe as a matter of fact.
Union Station is a nice building, but the railroad viaduct is definitely decrepit. The trackway closest to the building is basically splinters, and the outside trackway has been paved over. CTFasttrack might have proved useful as a ROW, but nope, they wanted buses.
84 runs both over (eastbound) and under (westbound) Asylum. The tracks pass under 84 on both ends. They'll have to close Church St, and probably take Spruce St., to get this done, which means they'll have to move the bus station somewhere else.
It's going to be an interesting squeeze.