Maybe not realised that London Underground had hand-push sliding doors into the 1950s, the last being those on the Circle Line, of all routes. It's easy for 3rd rail stock to have power doors, as they are constantly connected to the electricity, and also tended to have air brakes, which provides a good power source on each vehicle. The LMS Wirral Class 503, which I well recall, were built by the same outside contractors (Met-Cam and Birmingham RCW) as built Underground cars with power doors at the time. They had an emergency door release which was labelled that if operated you could then push the doors apart. They had the old Underground style of two air cylinders, small and large, connected to each door. The small one applied pressure all the time to keep the doors closed. The controls worked on the larger cylinder, which when operated overcame the smaller cylinder and pushed the doors open. Discharging the air from this at Doors Close meant the small cylinder closed them again.
The key issue was an absence of power source on traditional rolling stock, where locomotives were attached and detached as required. It was also, at the time, not perceived as any problem for people to open the doors themselves.
Paris Metro had hand push sliding doors into the mid-1980s on Classic stock. An air cylinder closed them just for departure, but then you could open them again. It was common in hot summer weather to run between stations with a door open, no different to traditional British buses with an open back entrance.
