Superliners are too tall to operate on the Northeast Corridor. The main problems are tunnels in Baltimore and New York, but also catenary is a bit low (I think the catenary was raised a bit on the track that the Capitol Limited uses on approach to Washington, DC). So, Superliners won't be running on the East Coast any time soon (except for the Auto Train, where they already run).
The Cardinal probably won't go back to Superliners soon (and, if it did, it would have to end in DC and not New York) mainly because Amtrak doesn't have the equipment to spare to put Superliners on there. That's why the Cardinal became single-level 2 1/2 years ago.
Horizon cars are mostly used in the Midwest. Some also make their way up and down the West Coast (including Pacific Surfliner trains 798/799 right now). Horizon food-service cars, and the occasional coach, can make their way onto eastern long-distance trains.