I'm not sure I would characterize MTH's Amtrak paint scheme as a "leftover." It's true that in 1971 they just added the black Amtrak logo to the demonstrators, which were 3-car sets, but they also supplemented these with added cars beginning in 1972. This paint scheme ran at least through 1973 on 5-car sets. Dated photos from 1976 show the red nose paint, so sometime between 1973 and 1976 they were repainted.
BRUCE CLOUETTE