fleeting
An automatic operation mode of an absolute signal that reserves a route for several trains following one another, without the need for dispatcher to re-set the route for each train.
fleeting: signal mode that auto-clears for train sequences
A fleeting is an automatic operating mode used in railway signalling systems where a single route reservation persists through multiple train movements without manual intervention by a signal operator or dispatcher. Once a signal is cleared for the first train in a sequence, the route remains held and ready to release the next train that approaches, provided no conflicting movements are queued on overlapping tracks.
The mechanism works by keeping the signal aspect at clear (or at least not requiring it to be manually reset to stop) after the first train has passed the signal, as long as the track section ahead remains unobstructed and no other train is being routed through an interfering path. When the following train occupies the signal's approach block, it automatically receives the clear signal without dispatcher action. Once that train clears the protected route, the signal typically reverts to stop or remains at caution, depending on system design.
Common variants and traffic patterns
Fleeting modes are most valuable on lines with regular, predictable traffic such as suburban commuter services or freight yards where trains of similar priority follow each other at set intervals. Modern signalling systems, particularly those using track circuits or axle counters, implement fleeting logic through relay logic or programmable controllers. The system must distinguish between consecutive trains of the same service and trains requiring different routes, which is why fleeting works best when traffic flows in one direction with consistent spacing.
The practical benefit is reduced signalling workload and faster throughput of repetitive train movements. However, fleeting can mask dispatcher awareness of individual train positions if poorly integrated with control systems, and it does not function correctly if trains bunch or disperse beyond design assumptions. Some older mechanical or relay-based installations may not support true fleeting, instead requiring a dispatcher to manually release signals for each train.
The term reflects the transient nature of the route reservation: it is not held indefinitely, but rather persists only for the duration that trains remain in close sequence. Once traffic flow breaks or a train deviates from the expected pattern, the fleeting condition expires and normal signal operation resumes.