While the Scarborough-York timetabled service in the 1970s was (in theory) thinner than today - 10 trains in each direction on a weekday and the last service out of Scarborough at 20.00 - I don't ever recall a service being cancelled on the route, and certainly I do not recall sustained reliability and punctuality issues stretching over more than a year and a half and counting.
What's more, the line was serviced by a variety of DMU types, plus on summer Saturdays a vast array of locomotive and stock classes. There was no massive hoo-ha and reasons not to run a different class down the route, as with the recent fuss over the proposed then dropped class 37s and Mk 3s. And again on summer Saturdays I don't recall services vanishing from the timetable despite the considerable demand for extra stock.
Oh, and DMUs were readily strengthened in the number of carriages, by doubling up units at busy times.