The 499 is sitting on the ash pit, so the cart must have something to do with cleaning fires and ash pans. Engines were not steam cleaned at the ash pit, that was done over by the roundhouse.
Back when I was cleaning fires, we were forbidden to run the air pump on the engine to keep the dust and dirt from the fire cleaning process from being inhaled by the air compressor. We'd shut the pump down, dump the grates, then clean out the pans with a "wand" attached to the main reservoir on the locomotive, using the air left in the system. Invariably, we would run out of air and have to start the pump, build up the air again, shut the pump down, then finish the job. I'm thinking this tank-on-a-cart. might be to store air for this process. But, it's not a very big tank and would hold a lot less air than the reservoirs on the engines would... Then again, ALA would have had shop air and water out by the ash pit.... so who knows.
BTW, the present SLRG engine house in which #18 is entombed is located exactly where 499 is sitting. The floor of the present engine house pit is the floor of the former ash pit.