Back in the day.....
Probably early eighties, the challenge was to ride on all the passenger lines in Cornwall on a day rover (£2.50 at the time). This was the four branches wholly within Cornwall and the main line Penzance to Saltash, the day rover didn't allow travel east of Saltash or travel before 9am.
I can't remember in which order we did it but it did involve quite a bit of doubling back, up & down the mainline, so the number used must have been in double figures.