Interesting that policy states which notches to use. You'd think they'd trust the judgement and skill of their drivers to use the notches they see fit!
I read that it developed in a similar way to the brake policy.
BR research determined a more efficient method after the original method was adopted.
When new, the driving technique for Springers was to apply a low power setting (at least Notch 2) to fill the torque converter at a stand, while holding Step 1 brake. As soon as the converter was filled, then straight to Notch 7 and then releasing the brake to give maximum acceleration.
At the time, this was deemed the most efficient method - getting to the required speed as quickly as possible.
Later, tests determined that this wasted a lot of fuel as well as not being particularly effective in poor rail conditions.
ScotRail determined in tests on their 150s that the most efficient policy was Start in 2, 4 at 5mph and Full power at 10mph.
Tests with then fairly new 156s determined the now widely used 3-5-7 policy was best.
I don't know quite how long the switch took, perhaps some drivers decided themselves to continue with the full power start method but it seems by the time 158s are introducing that the whack it open technique isn't the norm anymore.
I did recently watch a video on YouTube of Newport in 1989 with some 156s featuring (standing in for 155s) and they all seemed to be driven on the original Off-low-full technique.
