Interlocking
Domain 4 tests interlocking principles: how routes are established, what locking prevents conflicting or unsafe movements, how switch correspondence is proven, and how routes are released.
Function and locking types
An interlocking is an arrangement of signals, switches, and derails where the control of one is dependent on the position of the others. No route can be established, and no signal cleared, for a movement that conflicts with an established route or with a switch that is out of correspondence.
- Route locking holds switches and derails in position once a route is established and through the passage of the train.
- Sectional route locking releases portions of the route behind the train as it proceeds.
- Approach locking prevents the release of a route after a signal has been cleared (and a train may be approaching) until time has run - commonly several minutes - or track occupancy proves the move was made.
- Time locking requires a fixed interval, set by the railroad's special instructions, between restoring a signal to Stop and allowing the route to be altered.
- Detector locking requires track circuits within the route to be unoccupied before the route can be established.
- Traffic locking governs direction on a track signalled for both directions.
- Indication locking ensures the signal displays only the aspect that is actually proven in the field.
Switch correspondence and derails
A switch is in correspondence only when the points are within the required tolerance of the stock rail (about 1/4 inch) and the switch is locked. Circuit controllers must lose correspondence if points obstruct or move. Derails must be in the derailing position before a conflicting signal can clear, and they must be locked in the derailing position by the interlocking when the route does not require them clear.
Worked example
A dispatcher requests a route through switches 3 and 5 and the signal will not clear. Switch 5 shows reverse call but no correspondence indication. The interlocking is behaving correctly: route establishment requires both switches to indicate correspondence, so the control logic never picks up the route relay and never clears the signal. If instead switch 5 lost indication after the signal cleared, indication locking drives the signal to Stop and route locking holds the route; the operator cannot release it immediately but must use the approach-locking time release or wait for track occupancy to prove the move.
Traps
- Assuming a cleared signal remains cleared when a switch loses correspondence.
- Confusing approach locking (train-based, time release) with time locking (signal restoration based).
- Overlooking derails in the locking logic.
- Believing a route can be released instantly after the signal has been cleared and the train is approaching.
- Forgetting that material changes to interlocking circuitry require a Part 235 application.