Most steam operations on the mainline are supplied with water from road vehicles these days. Hydrants can suffer from low pressure and may be obstructed or full of dirt and crud. There are quite a few tanker operators now, from Blues and Twos mentioned above, to those who supply water for tasks from swimming pool filling to industrial users, building sites and major events.
In this case it was short notice and at an odd time of day, so a commercial provider would not be able to assist. While not an emergency, if the loco had run out the crew would have been looking at dropping the fire on the mainline and needing a loco to assist.