Three or four Amfleet coaches and one food service Amfleet would make a reasonable short distance train as used on the North East Corridor. I've seen such trains operating north from Washington DC.
I have quite a few of the Walthers Amfleets from recent years (my local hobby shop decided to sell them off for $10-$15) and they look good except for the silver wheels. They are (or were) available in different Amtrak schemes and both Amfleet I and Amfleet II body types.
The wheels are retained in the trucks by the lighting wipers (with or without lighting) and this increases their drag, compared to other Walthers cars.
I have a few of the original Bachmann cars which match the Walthers cars well enough to mix cars of both types, although they are slightly different arund the windows.
I'd agree that the new Bachmann cars look much better and I think they have less drag than the Walthers cars. I haven't bought any since I have more than enough of the Walthers cars from the bargain sale. Starting from new I'd get the new Bachmann cars, although Walthers have announced new Amfleet cars including some deluxe sets, but I'd guess you don't want to wait for those.
The biggest problem will be finding an Athearn P42 in Amtrak phase IV, if that's what you want...
Older schemes are easier to find, in my experience....
The Bachmann HHP-8 is really nice, and suits a North East Corridor train.