If it takes 4-5 hours to go from cold to ready, then the fire could be lit after midnight on the morning of the scheduled train, steam could be brought up, and a service day would not be used just building pressure. This would allow sufficient time for the locomotive to be warmed without causing undue stress and wear, and still have sufficient time.to do the start-of-day servicing and other preparation.
How much water is typically in a boiler when it is fired up from cold, and how long does it take for steam pressure to lift the needle off the peg once the fire is lit?
I'm going to the fireman's school later in the season, but I doubt this aspect of operation is covered in great detail. This procedure is one which is of essential importance, but doesn't get a whole lot of publicity, and is out of sight of most of the public.
Dave