The start from Rannoch northbound onto the 1 in 53 curving climb is quite hard, particularly for drivers of loco-hauled trains. As the speed limit is 15mph until the last vehicle of the formation has cleared the loop points, then 30mph over the viaduct, there’s not that much opportunity for the driver of long loco-hauled trains to get a run at the gradient (all trains, since 1988, have had to stop at Rannoch as it‘s a mandatory token exchange point for the RETB- back in the 70s and 80s, heavy freights could exchange tokens on the move passing through the station and get a better run at the gradient). A single 73/9 on load 6 with the sleeper (as was the norm in summer 2018 and 2019) would really struggle in damp conditions; one dreich morning in July 2019 I remember 73971 (working on three traction motors) slipping to a stand just north of the viaduct. The driver had to get permission to set back into Rannoch to have another go at the gradient. I seem to remember that he got up the bank OK a second time- after applying copious quantities of sand and turning off the ETS. It‘s a very difficult balancing act at low speeds with a wet rail, especially as the loco will not take full power in those circumstances.
Before anyone rubbishes the 73/9s, I’ve known the same happen to 37s in the same spot with poor railhead conditions (usually when the loco wasn’t in the best condition). I also remember a 67 (67009) slipping to a stand in lethal railhead conditions one autumn on the 1 in 70 leaving Blair Atholl with the Inverness sleeper.
