In reply to FesteringSore:
Heritage railways like the SVR still use electric token instruments interlocked with the bell telegraph instruments and the lever frame which operates the signals and points. As it happens, I'm told that the southern part of the SVR actually uses a different system called tokenless block which relies on track circuits to detect occupancy of a single track block section. OTOH I'm pretty sure that there are some low traffic sections of single track on the network railway which are still operated using physical tokens. UPDATE: just checked and the Heart of Wales line for one still works on electric token instruments, although they are controlled from one central signal box rather than the signal box at each station.
The West Highland Line uses RETB
https://en.wikipedia.org/wiki/Radio_Electronic_Token_Block which is fundamentally the same idea as physical tokens but uses radio to allocate and retrieve electronic tokens. There is no signalling on the line, apart from fixed distants on the approach to crossing points (IIRC you can see one of these alongside the line from the road as you approach Tyndrum from the North).
More modern signalling systems such as ERTMS
https://en.wikipedia.org/wiki/European_Rail_Traffic_Management_System are different again - ERTMS doesn't actually have lineside signals at all, it's all done by radio signalling direct in to the cab. That theoretically also allows an even greater degree of interlocking with the locomotive's control systems than is currently provided by AWS and TPWS (in the UK; other countries use different systems).
By the way, having now watched the start of the SVR programme, I do think the driver is being a bit of a bad workman blaming his tools. Flying Scotsman used to haul trains of twelve or more coaches happily up the 1:105 gradient through Gasworks Tunnel on departure from Kings Cross, after all. And after its recent overhaul it is almost literally a brand new locomotive (it was something of a 'Trigger's Broom' throughout its working life anyway - as were a great many other steam locos). I suspect he just had it in slightly the wrong gear, and he
might also have been a little unlucky in the position the pistons and valves had come to rest in when the loco was buffered up to the train (a bit like getting caught with your pedals in the wrong place for an uphill getaway on your fixie!) It seemed to work OK once the NRM's traction supervisor explained to him how to do it. As an SVR volunteer driver he probably wouldn't have had a great deal of experience on FS (unless he also drives for one of the mainline steam operators) and, as was suggested in the OP, driving a steam locomotive well does require a feel for the particular characteristics of the class, and sometimes of the locomotive itself.
For those who might be interested, the programme is being repeated on New Year's Day with the voiceover track removed, so all you get is the ambient sound/footplate chat.
Post edited at 15:33