It’s a journey I do every now and again, but it’s on a staff pass so doesn’t matter.
Travelling from Tattenham, unless you tap in as the doors are about to close, there are no delays, you run up the ramp at Caterham and there’s no queue at the barrier, it’s physically impossible to do tap in and tap out within 70 minutes.
The trains to Caterham from Purley used to be more frequent, but COVID stopped that.
In the other direction it should be possible within 70 minutes if there are no delays.