Shuttle Mode is a mode available for rides like the Monorail, which doesn't need a full circle of tracks. Instead, the train will move backwards once it reaches the last station or the end of the track. This enables the player to build transport rides in tight places or difficult terrain more easily.
Rides that can operate in shuttle mode include:
Roller Coaster Tycoon
Roller Coaster Tycoon 2
Roller Coaster Tycoon 3