In message , Tim Roll-Pickering
writes

Less than that - I remember it was once every 90 minutes, presumably because
the single track line didn't allow for passes (not without jamming the
Sutton to Balham line).

Immediately before electrification, the 1929 steam service was roughly
hourly, worked by a single train (20 minutes each way). But I'm pretty
sure there was another passing loop somewhere, because there was also a
fair bit of freight traffic on the line and some extensive sidings
around the area where IKEA is now situated.

Just after electrification the service was every 30 minutes (and every
20 minutes in the peak!), achieved with the aid of a third passing loop
at Waddon Marsh Halt and doubling some of the track at the Croydon end
(the whole line was constructed with a view to eventually being
double-tracked). But it didn't last ... not until Tramlink, anyway.

--
Paul Terry