I wonder why there's no serious talk about building over metro yards or highways? You look at the repair yard out on Eisenhower Ave or infill over 395. Boston does this like crazy, and DC has done it over 395. You'd have metro access and could even connect the streetscape. All things considered, I wouldn't think it adds that much cost to a $500 - 750 million project. The income from whatever rent Metro would help.
DC reportedly considered doing that for what became Nats Park over I-395 down near the Southwest Waterfront (roughly between the overpass leading to the 12th Street Tunnel and the overpass coming from the 9th Street Tunnel). Apparently the cost was deemed prohibitive.
