Magic Kingdom Railroad Train

[Robo]

And the train doesn’t run during the parade. We once boarded the train at Fantasyland then sat for 40 minutes before we could go.

You can see the reason why in the image below (far right.)
The parade must cross the RR tracks as it enters the park at the end of the Frontierland street.
So, since all of the trains in operation are on the same track, one following another, they all must hold when the parade is crossing the tracks.