OK, so what happens now is that the railroad crosses from the west side of the expressway to the east side for Hartford, and then crosses back to the west side. The plan is to put the railroad line entirely on the west side. The highway is half-elevated and half-depressed, and the plan is to have it fully depressed. With the depressed railway line on the west side of it.
When this eventually gets built, the new below-grade platform (double track, I don't know whether they're going to do island platform or side platforms) is going to be roughly a block from the existing station -- less distance than the Sacramento walk -- about 400 feet. (And an elevator or stairs down.) Furthermore, I believe the plan involves the closure of Spruce Street, so with Spruce Street closed and the highway buried, it could be a direct walk. So they might actually keep the existing station building. More likely they will build a new station building and convert the existing building to something (it may remain the bus terminal).
The project is very expensive and so it may not happen for a long time.
....edited upon reviewing updated information. So they're going to be moving everything further west than I thought. Based on current plans, it looks like it'll be about 900 ft. The Capitol View Apartment Homes will be demolished and the train tracks and platform will be right under there. The two surviving buildings on Spring Street will also be demolished and the highway will run under there (the railway will be to the west of there).
Latest info at the March 1 presentation here:
http://www.i84hartford.com/working_group-Transit_Tech_Comm_materials.html
Oh -- also it looks like they're settling on the four-track option for the station.
http://www.courant.com/news/connecticut/hc-viaduct-community-meeting-20170420-story.html
The consensus seems to be to convert Union Station to some other purpose (events, probably) once the new station is open.
I'm OK with that. Four tracks, two platforms, grade separated, a couple of blocks from Union Station -- pretty good.
Oh, the project would also straighten out the tracks so the trains can run faster.