Amtrak switches cars between the SUNSET and TEXAS EAGLE in San Antonio.
In the pre Amtrak era, it depended on the route and the train. Before Amtrak, the UP would switch one huge passenger train from Chicago into three trains. One part came off at Green River WY, and went to Portland OR. Odgen UT divided the rest of the train into a LA and SF train.
MoPac had a like operation in NE Texas for the branches of the TEXAS EAGLE. SCL did likewise for the east coast trains to/from FL.
Lesser mail and express trains picked up and dropped cars almost at every major point.
Part of the fun is reading and studying this stuff.