This is for the standard looping and player drivable train where you can TAB into.
<fs19_root>\data\placeables\mapUS\trainSystem\trainSystem.i3d contains transformgroup trainSystem which has one hidden spline called track01, then railroadCrossing01 through 09.
<item mapBoundId="trainSystem" className="TrainPlaceable" filename="data/placeables/mapUS/trainSystem/trainSystem.xml" position="0 0 0" rotation="0 0 0" />
- create transformgroup trainSystem, do not move it, leave it to 0,0,0 coordinates
- create spline called track01 into trainSystem transformgroup, make it hidden by setting visibility off
- when finished creating track01 control vertexes, close the loop with O key
- select trainSystem export selection, save it as trainSystem.i3d, then delete trainSystem transformgroup
- place trainSystem.xml next to your trainSystem.i3d file
trainSystem train does work with open ended spline (not closed loop). Once train reaches the end point, it teleports to start point. This causes no error in the log.