This seems to be part of the basic set - I started a new game with no mods and it shows 20 of the 21 models. Not sure which one it may have been... or may be just the total number is miscalculated and there is nothing in reality that is missing.

@adlet I've just replicated what you described in a new save. Clearly the missing train type is not related to the issue. One of the trains that I've always purchased as a pre-configured one. isn't available from the menu (or from the custom train menu). I can build it as a clone though (but then it is listed as a custom train).

Curious which one, as I see 20 lines in custom trains and 20 types in preconfigured, and they seem to match. I think also some trains are only supposed to operate as sets with others so they would not show in preconfigured standalone.

It happened with a mod. It was there as a pre-configured train, now it isn't (it's clearly in-game because I can't disable it). It's not available in the custom list either but when I clone an existing one it goes to the custom train menu.

If it is from a mod you don't use anymore, did you try to remove orphaned rules?

The mod is still in the save just not available from either purchase menu.