Doc is the Kahlville Works' first responder engine, immediately at the scene the moment word of trouble comes to his yard.
It's no time at all before Doc gives the engine (or wagon) in trouble a good checkover. A few tests later, and he's got whatever they're going through diagnosed! Doc isn't able to take trains for them, as he needs to get them back to the Works for repairs or home if they need minor ones, but he's proved time and again to be an invaluable asset to the Kahlville engine community.
No matter the trouble, Doc's on the case!
Doc lives in the Kahlville Works, sleeping in his own personal siding.
Fun Fact!
Doc will sometimes be assisted by Mary, especially if special equipment is required to be fitted to the patient!