That’s just a US thing. In Europe, it’s routine for trains to couple or uncouple in the middle of a journey. For example, the 8-car train from London to King’s Lynn will leave 4 of its cars in Cambridge. The uncoupling process (and the coupling process in the other direction) takes less than 2 minutes.
On a bigger scale, I once boarded a night train in Venice half of which was going to Munich and the other half to Vienna. When I arrived, some of the cars in the train I arrived on had come from Zagreb and Budapest.
