Many thanks. I'm guessing that if these few trains call at Redhill, there are no trains that call at Purley and then take the Quarry line? What about the overnight ones that are non-stop from Purley to Horley?
They can take either the Redhill Mainlines or Quarry Line, except for the staff trains which call at Redhill, which obviously can't use the Quarry unless there's no other option. Timings should accommodate either without too much hassle. Overnight trains can use Platforms 1 & 2 at Purley, but it just depends what's open and when.
As a point of trivia, the other night there was a block on the Down line near Redhill station and a block of the Up Quarry. Therefore most overnight trains ran via Redhill Northbound and the Quarry Southbound. However, because of a block on the Up Main (through line) at Redhill, a tiny number of Northbound trains passed through Redhill Platform 1 non-stop. That's fairly rare, really. Unfortunately I didn't see what was happening as I was only there at 0400.
If there is only one line open between Redhill and Purley Junction (just North of Purley), and the Quarry Line is closed, pilotmen can guide trains between Redhill and East Croydon, meaning they will change at Redhill. This means all services may call at Redhill overnight - it happens a couple of times a year.