A bain-marie keeps food hot by sitting it in a heated water bath. Water spreads heat gently and evenly, so the food does not catch at the bottom; the same principle serves for melting chocolate and for holding pans on a self-service line.
On the line a bain-marie is a holding device and is not designed to bring a cooled pot back up to a safe temperature. It also depends on electricity: with no socket in the vehicle, the water holds heat for a while, then drifts towards ambient and pulls the food down with it.
On the road the work is done by insulation. Food goes into an insulated container at service temperature, the lid closes, and the journey can last as long as the container holds that heat. Departure and delivery times are therefore planned together; if food waits between delivery and service, a bain-marie in a powered area becomes useful again.