Paul Corfield wrote:

Yes and where do they then go? Moorgate is a very tight bottleneck

Really? Has its capacity been reduced since all that extra stock now on the
North London Line used to run there?

It may be poor but I do use the Hertford line infrequently and
typically off peak. It is never more than 20% full even with 3 car
trains.

.... because people who have a choice use the Piccadilly Line, because it's
more frequent. Too frequent, actually - north of Arnos Grove, the Picc seems
to have about 1 person per carriage for much of the week.

Would TfL really run that level service parallel to a 18tph tube
line? I think you've just answered your own question.

A train ever ten minutes all day south of Gordon Hill and New Barnet would
do a lot to pull people back from the Picc, so frequent trains might end up
fuller than infrequent ones. Wimbledon to West Croydon springs to mind.