There are several Couple driver commands.
The built-in Couple command lets you choose a specific car anywhere on the route. When the command is exicuted, the train, under AI, will drive to where ever the car may be and couple to it.
I prefer Couple at TrackMark - it lets you couple to whatever car may be at a given trackmark. You do not need to know what the specific car is.
THere is also couple at signal - similar to Couple at TrackMark.
THere may be others.