As a regular Dorking-Waterloo commuter, I find it really frustrating that SWT relies on Southern services to pick up passengers when it wants to clear its timetable and cancels the train at Leatherhead, 8 minutes from the final stop.
This only happens when the trains are already running very late, and therefore adds to the delay of the passengers trying to get to Dorking, as they wait for a Southern train to arrive (can be up to 30 mins).
I understand that congestion needs to be cleared, but the message it sends is that your service timetable is more important than your passengers.
Turning trains back short of destination isn't desireable.
It does mean that the next working will be on time (in this case from Leatherhead) and hopefully on time and in the correct path into Waterloo. That means the stock and crews are in the correct place for later workings.
So it comes down to a balance, 20 people delayed at the far end of the working or the next working carrying 10 times that number being delayed and further workings and may more people delayed.
I'm afraid that the days of a crewed spare train sat at Waterloo to cover for late running have long since gone - basically down to increasing traffic and getting the maximum work out of resources such as rolling stock, platform occupation and most importantly for the accountants the train crews.
Disappointing to find this happening again last night. A train already running 40 minutes late cancelled before the end of the line is very dispiriting for the passengers who have been waiting for a driver to arrive so that they can leave Waterloo. I suppose there is a sense of inevitability that our journey time will be extended even further by a cancellation when a train is running that late, but that doesn't help manage the frustration felt.
The maddest thing about it, keeping in mind the points raised by Romsey above, is that it takes over 10 minutes for the driver to move round to the other end of the train, and for the guard to check everyone's out. By which time the train could have completed its journey and been heading back to London. Again, hard to escape the sense that those passengers trying to get to Dorking are somehow less important than others.