The number of trains which divide and conjoin are few and far between these days. It is a very labour intensive process to attach and detach coaches to/from trains and you have the added problem of what to do when a train "providing" a through coach to another train arrives late - does the second train wait, and also get delayed, or, if it doesn't wait, how does the through coach get to its destination?

Examples of ones that still exist can mainly be found in Eastern Europe - e.g. overnight trains to Russia.

Additionally, most long-distance trains are now self-contained units - ICE, Thalys, Eurostar etc - and you just can't add a coach at will.