The obvious solution would be to pick one locomotive, perhaps the first one, the last one to be retired, or one that was the most photographed around a large city, and build a model of just that one locomotive. If that isn't good enough for too-picky fans, they don't have to buy it and probably don't deserve to have anything. I doubt that any of my O gauge locomotive models from Sunset, MTH Premier, Lionel, Atlas O or Weaver is precisely accurate - but some are quite close and that's good enough for me. Remember, in building any locomotive model, deviations from prototype always must be made to engineer the model for operation. They never are perfectly accurate - just close enough for most people.
I disagree. What if a customer wants to purchase 2 or more models, with different road numbers, for "his/her railroad" being modeled? Then, what if a customer wants correct models of units used in freight service, plus additional correct models of units used in passenger service? Also wanting to purchase multiple models? Sure, it would be expensive but, what in model railroading isn't expensive any more?