Oddly enough I caught a train around that time last September from Romiley to Edale. 2 car pacer, had to stand. No doubt if you had arranged for all the passengers to squash up on every seat those standing would have been able to sit down, but for me standing was OK.
Weather wasn’t special, it rained in Edale, and a dog society were taking all their hounds for a train ride too so a dozen dogs and owners helped fill the train.
Of course that’s just a one Sunday snapshot, someone more local may have better information.