What's the (approximate – I'm not expecting an exact answer) highest capacity of passengers a train has managed on a preserved line in the UK? What's the highest theoretical capacity possible with current rolling stock in preservation?
Related questions would be what the longest train in preservation has been, and, more tangentially, what the longest platform at a preserved station is.
Given that nearly all preserved railway is single track, length of run round and passing loops are an important constraint on train length.
I haven't travelled on preserved railways since covid, so things may have changed, but the railway I went to expecting 8 car trains at galas was the Severn Valley. I have a vague recollection that, at one time, the East Lancs also ran 8 car trains, but possibly only before the Heywood extension opened.
The longest train I can recall travelling on was 11 cars on the West Somerset when a 7 car special and a 4 car timetable train were combined for the last departure out of Minehead.
For packing lots of passengers into a small space the quad art set at the North Norfolk will be hard to beat, though I can't find exact figures for seat numbers.
