The Mt. Rainier Scenic's red coaches are steam heated. The crews I've talked to confirm that heating the cars uses up lots of steam.
It's a real balancing act to get the heat right. The first coach behind the locomotive is warmest (often hot), and the heat available in the steam line drops with each coach. At the end of the day, the line has to be carefully blown out to prevent any water from remaining in the line to freeze.