You can mod every detail of any vehicle in the game. You can open with Notepad the corresponding .mdl file and change the specs you want. That’s what I did for Tank Wagons for example – reduced capacity and maintenance cost, so more wagons are needed for the same amount of cargo, thus making trains longer and keeping yearly costs the same.
I don’t remember the exact path to model files, but it should be something like ..\res\model\models\vehicle\train\.
Also one of the latest game patches introduced simplified modding – you can script your mods in the \mods folder and enable/disable them dynamically inside the game. I think they released a special documentation on that.