Texgeekboy, I hope you find a suitable engine.
Besides the advertised minimum radius for the locomotive, you should also consider the length of the locomotive, because, not only does the locomotive have to negotiate a sharp curve, but it has to also not pull the car behind it off the track. Diesels like a GP40 should be no problem, but the longer 6-axle locomotives might make it around the curve and derail the car they are coupled to.