I just started playing Transport Fever last week, and then I discovered this. Looks amazing! 
I need to play the game some more, but when that has died down a bit, I think I want to get into modding myself. I'm a member of the ULJ (Upsala-Lenna Järnväg) museum railway, I happen to be pretty good at modelling things in Blender, and I think my ambition would be to eventually have our entire fleet of active locomotives (and train cars) in the game.
That feels like an ambitious goal though, seeing as we have three running diesel locomotives (z4p, z6p, tp3515), a bunch of railbuses, 4 steam locomotives (BLJ 5 Thor, BLJ 4 Långshyttan active, NÖJ 16 active within the next few years, SRJ 28 Stortysken next up for renovation), and a varied assortment of train cars.
It would be awesome though, so I'm going to try to make it happen!
A minor snag is that the ULJ track is 891mm narrow gauge, and the game doesn't seem to make that distinction, so the proportions might get weird if we were to put them side-by-side with modern standard gauge trains.
Well, in any case, I'm off to play some more. I'll be checking back here regularly, and then I'll try to get into the technicalities of making at least some of it happen over this year 
