You put it in the editor, but there's a complicated process along with this "Mighty" mod, the train installation requires Unity game editor and maker. Go ahead, search up a tutorial or two.

Goto said map you want train. Look for a Config.json notepad thing. If none add New Text Document, enter text document, press Save as:Name it Config.json. Once saved open it. And add this text:{
"Trains":
[
{
"VehicleID": 18690,
"RoadIndex": (Index of the road/tack)
}
]
}

Now save it and load the said map.