I dont know if this helps past posts or future searchers but anyway. I too had struggles with spawning spline and actor then make actor use spline points as travel paths.
My issues were I was trying to call my combo within a BP then on to the level.
YOU MUST in this case Make clear Variable References to the Track(Spline) in the Cart(Actor) -the track has nothing inside this BP besides a Spline component.
Your Moving Actor uses the refence create of the spline to target this specific track right after SpawnActor. You must link your SetRef of Spline From the return value of the spawned spine.
I created a timeline within the Cart to set playrate/speed etc.
if you need help follow Cart and Track videos on uTube.