Gas lighting and oil lamps.

On the trains themselves, especially passenger trains, many of the older coaches had, beneath them, two large black gas cylinders bolted to the frames. From this was fueled all the lighting in the carriages and the compartments. These were still in use during the 1910s, and helped to contribute to the greatest rail disaster in British history, the Quintinshill Disaster

Freight trains were rarely lit, and those sat on the footplate of the locos were lit by oil lamps and the firebox itself.

What was lit during the night were the signals. Some of the early binary signals were not lit, and this was almost entirely pointless. What developed afterwards were signals that would work in the dark, using red and green gels and oil lamps.