Shuttle system. The duration of each sequence can be adjusted from approx 20-70 seconds.
Gaugemaster SS1 Super Shuttle Unit - Electronic Shuttle Control Module
N Gauge Le Shuttle CJM Single deck carrier shuttle wagon Eurotunnel Code E349
Great Western Railway - System Map - GWR - Map of System
Shuttle train contoller
BLOCKsignalling SS2A Model Railway DC Shuttle Controller with Speed Control