As Mac Gaddis and others have told me, there would be a caboose supply building at any yard where cabooses were serviced. This might be part of an entire complex of small, mismatched and sometimes ramshackle buildings, and might even be a box car or other car body incorporated into the complex. There had to be storage space for all the supplies, and working space for the assigned employees. This kind of building would be painted Colonial Yellow with Light Brown trim.
The photo immediately below, taken at the West Oakland caboose track on October 11, 1946, shows two features. At left is a water tap, supported by and protected by a wood post (and note the Allied truck), and in front of the truck of caboose 205 (Class C-30-3) at right are several cloth bags of coal. This is how coal was supplied. (The R.W. Biermann photo is courtesy of Arnold Menke.)
The next photo is very informative. It shows two service carts, the one at right apparently battery powered. The one at left carries ice and bottled water, while on the one at right are cartons of Olin fusees and Texaco “hot box coolant.” This photo is from the Roseville caboose track in 1962, and is SP photo N-6514. The caboose at center is number 1191, Class C-40-3. Both these photos (cropped less severely) are in my caboose book (Southern Pacific Freight Cars, Vol. 2, Cabooses, Signature Press, Berkeley and Wilton, 2002).
My branch junction at Shumala has a caboose track, so I need to provide a small building for caboose servicing. An old box car body, perhaps one of the fine Westerfield models of early Harriman box cars, too old even to be in MOW service by 1953, might make a good choice. I’ll probably position a few bags of coal outside.