But the preparation to be ready to do that takes time. Here there are two important differences between Horsham and Cambridge, and I have watched splits at both.
One is that the churn of passengers at Cambridge is much bigger than at Horsham. When a train splits at Horsham, most passengers are staying in their seats, with only a few alighting and boarding. When a train splits at Cambridge there are lots of Kings Cross-Cambridge passengers getting off and lots of Cambridge-Fen Line passengers getting on, with conflicting movements at the train doors and on the platform.
The other is that at Horsham both portions are continuing in passenger service, so there is no requirement to empty out the rear portion before splitting. At Horsham one stage of the process is missed out completely, thus saving time. At Cambridge the rear portion, arriving very full, has to be emptied out before the doors are closed so that the actual split can be done. There are always stragglers in the rear portion, going forward into the Fens, who are oblivious to what is going on around them and only move to the front portion after they have received the verbal equivalent of a cattle prod from the platform staff.