To add to this:
The Märklin designers had many smart ideas. They also made in 1906 a so called Heizwagen (heating car) in gauge one which just was a boiler on wheels. It included some tubing with leaking connections such that you could heat your passenger cars and showing steam coming from the connections. https://www.historytoy.com/mar...ng-car-gr3en-gauge-1
It was based on a real steam heating car used in one of the first train running in winter with an electric locomotive in Germany.
Regards
Fred