Staff stops at Selhurst or Norwood Junction (depending on the route an overnight train uses) are still very much alive and well. The stops on TL trains have historically been a much more closely-guarded secret for many years, but have become somewhat more necessary since the overnight Southern services were slashed for whatever actual reason (a sorry situation which allegedly was only temporarily, but in reality has become a near enough permanent absence on weeknights now).
Most such stops are made as per the crew diagrams, and as such they are generally not made on an “unofficial” basis in any way, especially given doors are usually released at staff stops at Selhurst in particular (though do not have to be) and it would be foolish for someone to randomly let passengers on and off trains without official consent.
Special Stop Orders can be given which specifically dictate staff stops are to be made, and in such cases, Control will also state whether or not passengers may use them if they so desire.
You can also have Crew Stops, which would be for the specific use of train crew only (not just any and all staff, which can include cleaners, fitters, managers going to the office, etc.). These stops usually involve use of cab doors by competent persons, and no door release is given for passengers to board or alight. Again, these can also be done as per diagrams or if a Special Stop Order is given.
Some ECS moves have been known to stop unofficially for crew change purposes at Selhurst or similar, and this is not generally ever frowned upon, though this is rather more by local custom than official consent.
The matters have been muddled over the years by things like station locking-up policies meaning doors on the train can/cannot be released, or by very restrictive SDO on some fleets, meaning some passengers may not even realise the train may have had its doors open in a different carriage.
As for stopping the odd GatEx specially - been there, done that! I did experience one unfortunate occasion where a driver said “only one staff stop please, or I’ll miss my train home” - so I persuade him that stop x was the most urgent as an Important Train would otherwise be irretrievably cancelled. I alight at stop x and observe that the driver accelerates rapidly towards London, no doubt thinking he had made up time, and would shortly be on his train home... said train having just been cancelled because other crew had been displaced by a further problem elsewhere! The poor people at Gatwick who were wanting all the other intermediate stops could have slowed the GX down no end with their requests, and the driver would have got home at just exactly the same time.
- and the ones in the morning are to facilitate connections from the Marston Vale line from Bletchley to Milton Keynes Central - I don't think any crew changes are booked (same on the xx07 off Milton Keynes Central to Euston) but yes they certainly do happen!
