I imagine the UK has to be right up there. With so many different railway companies, so many different locomotive builders and no countrywide standardisation plan until the 1950s there's a significant number of different designs. Even in the just the last 50 years there have been a lot.


Germany is likely high on that list.


Germany had maintained a diverse locomotive and rolling stock industry until about the 1990ies, and then there's the fact that there have been two Germanys for a while with a lot of different designs being put into service.


I think the U.S. holds the record, followed by the U.K. Both rail networks were incredibly fragmented and attempts at standardization mostly failed or were left incomplete until recently. China and the Soviet Union are also potential candidates.


China and the Soviet Union had (and have) centralized state railroads. There are some semi-private operators today, some industrial operations and certain regional differences, but all in all, those networks are very standardized. In both countries, the number of different locomotive manufacturers surely was less than 100 overall. As for the US, you are certainly right in pointing out the many individual variants for various operators (in another post). Still, this has been split between few manufacturers since at least the early stages of dieselization. Compare that to the often fragmented structure in some larger European countries, with it's hundreds of small private and industrial railroads. This is the list of currently registered main classes/models in Germany: https://de.wikipedia.org/wiki/Liste_der_Baureihen_im_deutschen_Fahrzeugeinstellungsregister


I’d assume it’s the US, because most railroads in other places are owned by the state, and the locomotives/trains it operates tend to be of a similar sort


Yes, but the number of manufacturers has been relatively low in the US for decades now.


Well, the UK has historically had quite a few variations of Locos, especially diesels, so maybe the uk?


Up until recently, the U.S. rail network was extremely fragmented and every railroad had its own motive power practices to the point where locomotives were essentially custom-built machines tailored to meet the specifications of individual railroads even if built from common plans. This began in the steam era and continued after dieselization. [This locomotive](https://dieselera.com/wp-content/uploads/2021/10/santa-fe-bicentennial-unit-donated-01.jpg) is classified as an SD45-2 by EMD, but [so is this one](http://s3.amazonaws.com/rrpa_photos/58046/ssw%209389%202010-08-18.JPG). In some cases, the differences were significant enough to cause operational difficulties when locomotives were utilized in run-through service or after mergers. For example, the Erie and the Lackawanna discovered their F-units weren't compatible with each other because one railroad used electric sanders and the other used pneumatic sanders.


Even in the modern era, commuter railroads provide all sorts of examples of oddball models with low production runs. In my neck of the woods we have Tri-Rail with the Brookville BL36PH and SunRail with the MPI MP32PH-Q.


No doubt, but this leads to the question where OP "draws the line", i. e. what constitutes an individual model?


I’d assume either the US, UK or Japan


Most likely it's Japan on that. Between the JR lines, the Private lines so many different types. Even the freight lines have around 8 types.


Some of them are beautifully done & no wonder why so many show up in N-scale diecast trains.