I might have more specific info on the current UP operation once I get out there, but in general, when steamers operate on main line excursions, fuel and water are typically trucked to pre-arranged servicing locations. On smaller steamers, local fire departments are often engaged to water the locomotives with some sort of fee paid for the work. UP may have hydrants available at some sites. I see what appear to be hydrant connections on the tender. Oil deliveries are not unlike getting a delivery of home heating oil at a house, except the product being supplied is typically waste oil (basically used motor oil) and in this case, the truck needs to be bigger.
No rocket science involved. It just requires someone arranging the logistics for the deliveries.
/Kevin Madore