How far did you get with your mine cart vehicle? I only ask because I just made a mine cart demo. If you still need tips on how to implement you can ask me. But the easiest way is to set the passage options of the rail tiles in the tileset settings so that you cannot go off the track (passage 4-way settings). Then you just have a mine cart event that you interact with and it changes your player into a mine cart. You can then move and hide the mine cart event so it's out of the way. You can have track exit nodes that turn your player back into the player.