I believe it was after merging with the Shuttle US decided that using a dedicated group of planes didn't make sense - what did Eastern know anyway. So an A320 might fly CLT-LGA then a Shuttle segment then back into the general system. After finding that NE delays then rippled throughout the US network US decided that maybe Eastern knew what they were doing after all and went back to a semi-dedicated fleet where a plane might spend an entire day doing Shuttle ops before/after flying in the general network.

One thing about the pre-HP US was that they had to unlearn a lot of "it's the way we've always done it" tricks.

Jim