John,
The Huckleberry RR operates during the Christmas season, at temperatures as low as -15 degrees Farenheit! They use RV type gas furnaces. These replace one end seat and fit in a box about eighteen inches high and two feet square. They require one inconspicuous vent in the car side and use flexible ducting under the car floor, leading to vents under the seats. The ducting is the most visible element.
The D&RGW cars were heated with Baker Heaters, which were simply coal fired heaters that cirulated hot water by convection. No pumps or electricity required. There must be a modern, gas fired equivalent out there somewhere. The water piping would be less noticeable than the air ducts that the Huckleberry uses.
Check out a photo of the Huckleberry in winter at the top of the Goat. You won't notice the side vent or the ducting. It was seven degrees that day.