It can be done very quickly with multiple units (powered carriages without a separate locomotive).
In the UK this is known as "portion working" and is a routine part of some routes' timetables.
The slight difficulty doesn't come from the separation, which is easy - open the automatic coupling, drive away the front bit and then drive away the rear bit after the front has got clear - but the joining. With an intensive timetable, waiting for a (possibly slightly late) other portion can leave a train missing its booked slot into London, with knock-on delays.
In the UK this is known as "portion working" and is a routine part of some routes' timetables.
The slight difficulty doesn't come from the separation, which is easy - open the automatic coupling, drive away the front bit and then drive away the rear bit after the front has got clear - but the joining. With an intensive timetable, waiting for a (possibly slightly late) other portion can leave a train missing its booked slot into London, with knock-on delays.