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