That is an amazing film...That is one of the seminal railway-porn films!
I seem to recall reading that the 1957 Lewisham rail crash was partly attributed to difficulty reading signals from the cab of a Bullied Pacific loco and hastened the roll out of AWS across the national network.
In steam days firemen were certainly competent to read signals and would assist the driver by looking out of the offside of the cab to do so.
It’s often struck me when reading reports about crashes in steam days that SPADS sometimes happened because drivers had passed through the same set of signals time after time when they’d always had a clear road, and a momentary lapse of concentration the one time in a hundred that it’d been different had ended up costing them dearly...
There were occasions that drifting steam would have caused problems, but there were also occasions when drifting steam was blamed by a driver that might have just as easily lost concentration due to the loco steaming badly, being under time pressure, or being heavily fatigued due to some of the extremely arduous shift work that was put in back then in very difficult conditions.
There were of course plenty of accidents that lead to the loss of both of the footplate crew which meant that some of the questions remained forever unanswered...