Well if you could only get 30 minutes or so, I don't think it would be worth the trouble and expense. Think in terms of at least 2 hours, and more likely 4-5 hours or more. It depends on the current draw of the individual locomotive and how much room you have for batteries. For locos that draw 1/2 to 1 amp, I try to use 4000 to 4500 mah NiMh batteries, which will yield that 4+ hours of runtime.
The guys that use drill batteries usually get their long run times by swapping out batteries, as opposed to charging in place. But I would still think 2 hours at least? And I may be wrong, but I get the impression that most folks using drill batteries are running power hogging multi-motored diesels.