Higher center of gravity, Superliners are higher than the Santa Fe cars. I've also noticed that trackage on the former BN side of the BNSF has gotten much rougher since the BNSF merger, rode the Empire Builder from Chicago to Portland and 10 years ago was a nice ride but now, some places are simply rotten! Spokane to Wishram was really rough, used to be a nice ride on those concrete ties but no more!!