Initial shots were of a class 20, interior shots of mark 1 stock and the initial shots from the inside of the train suggested a Southern Railway station. An 08 or 09 shunter also seen, propekking a tank wagon. A number of semaphore signals seen including shunt signals, leads me to think the Mid Hants Railway might perhaps be the location. Not quite right for the period as class 20s did not haul passenger trains between Chelmsford and Liverpoo St. At the time any main line engine would have been double manned - and releasing the controls and slumping unconcious would have resulted in brakes being applied. At least it was mark 1 stock - correct for the period.
I have watch the programme since it started. I grew up in the '50s and '60s and enjoy it because so far it has been a pretty accurate history of the NHS and surrounding issues that impacted it, (smallpox, thalidomide, measels, abortion law, homosexual bill etc.). In fact so far the main inaccuracies have been the cleanliness of the streets - in Poplar?, and the rugby passes that babies are put through owning to social distancing (by the actors).
Today's episode I'm afraid was well below it's normal standard. In 1968, a train journey to Chelmsford would probably be via Liverpool St. as very few trains would start in Fenchurch St and pass through Poplar (not sure which line that would have been on the bridge, - the Blackwall Railway now DLR? So the journey would be from Liverpool St probably in a class 309, or maybe a 305/308. Whatever it was, a class 20 wouldn't have pulled anything other than ECS. MKI coaches were OK but the 309 interior was far more modern than that.
Then the crash, the driver presumably had an epileptic fit. The AWS would have acted when it wasn't cancelled at the first signal, - it probably wouldn't have reached a second signal.
It was probably planned and executed by the team that does the annual train/plane/motorway disaster on Casualty!