Actually, the tunnel under DC into Union Station from the south will clear a Superliner. And in fact, until the Cardinal went to single level cars several years ago, it routinely operated into DC using Superliner cars. My first ride on the Cardinal was Chicago to DC on Superliner equipment, and IMHO, a much better way to see New River Gorge when there is a Sightseer Lounge in the consist.

It's the other tunnels further north, namely at Baltimore, the North River tunnels (Hudson), and the East River tunnels that will not clear a Superliner.

I'm not aware of any bridges however that will not clear a Superliner; not saying that there aren't any, but I've never heard that cited as a reason. There are places where the catenary is too low, but that could be rectified rather easily.

The two biggest reasons for no Superliners in NY Penn remain the old tunnels that will not clear the cars and a lack low level platforms for boarding.

Therefore any long distance trains that operate into and out of NYP, must remain single level consists.