Los Angeles' Union Station (built 1939) is often called "America's last big city train station", but with today's ongoing reinvestment in rail travel it seems to me that title will not be accurate much longer. Lots of big cities are expanding, upgrading, or building their stations. So many that it seems like a boom. Let's share the plans/proposals here.
Here's the list of stations included in this thread so far. I'll edit this list as more are included.
Here are the ones I know about. Surely there are more.
New York Moynihan Station
Proposal to replace existing Penn Station. They would reconceive the historic post office building next door, gutting it and rebuilding it as a train station. Handles Amtrak (including Acela), as well as local regional/commuter rail.
Website.
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Washington Union Station
Expansion of existing building, to replace the train room. Handles Amtrak (including Acela), as well as local regional/commuter rail.
Website.
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San Francisco Transbay Terminal
Reconstruction of an existing bus station to handle future California High Speed Rail, as well as local regional/commuter rail.
Website.
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Atlanta MultiModal Passenger Terminal (MMPT)
Future new intercity rail station.
Website.
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Denver Union Station
Addition of a new train room to the vintage depot, primarily for regional/commuter trains, but also for Amtrak and trains from Denver to ski resorts.
Website.