Most model manufacturers (the ones that market towards model railroaders, not toy trains) are doing quite a good job of accurately numbering their offerings. There was a time when even the likes of an Athearn would slap any road name on a model, sometimes regardless of whether that road even owned it or not. Nowadays, there's even a lot of instances of the model builders changing details like headlights, etc. for each specific road.
For locomotives (the real ones), they all come from the builders with builders numbers, which are akin to a serial number on any other product, which is selected by the manufacturer. That's how one specific unit could be tracked, as the road number will change anytime the railroad sells the unit...
Wondering if I'll see the Western Route double-tracking finished before I retire...
Photo: Melbourne W7 No. 1019 on Route 78, Bridge & Church Streets, Richmond, Victoria. 10/21/2010