Signal Overlaps

This page uses a sequence of photographs to demonstrate signal overlaps and the operation of automatic running signals.

What Is An Overlap?

Overlaps provide a safety margin should a train overrun a signal at Stop. The Railway Encyclopaedia, EF Carter, Harold Starke Ltd 1963 defines overlap as follows:

In signalling practice the distance beyond section "B" which a train must travel before a train following can be allowed to leave the previous section, "A", assuming that sections "A" and "B" are adjacent and trains run from "A" to "B".

Overlaps can also be explained with an extract from the NZGR Rules and Regulations 1965:

NZGR Regulations For Automatic Signalling (1965)

The first page (reproduced here) is a description of three aspect automatic signalling.

Scroll down for an explanation of overlaps...........

The third paragraph from the bottom implies that as soon as the train at "B" has completely passed "H", signal "G" will no longer be held at red. This is the case if there are no overlaps. If, however, "H" has an overlap of, say 200m, then "G" will remain at red until the whole of the train has passed 200m beyond "H".

Where Were The Photographs Taken?

The photographs were taken in the Hutt Valley, near Wellington New Zealand. The following diagram shows the relative locations of the signals and of the photographer:


Here is a set of six photographs showing the operation of automatic three aspect signals as a train passes. The fourth photograph in the set shows two signals at Red with a train in advance of the more distant signal - an overlap.

Despite taking the same sequence of photographs ten times I am not entirely happy with the results. I have therefore picked a set which shows all of the signal colours and added a seventh photograph from another set which better demonstrates the overlap. I had difficulty getting a green signal in advance to show on the photographs. If the signal heads are one day replaced with LED units I may re-take the photographs...

Clear, Normal Speed Signal

80 Down Starting From Down Main (Waterloo) {closest to camera} and 74 Down Directing (Woburn) {in the distance} both show Green over Red.

The "A" light is on which means that Woburn is switched out and the signals are operating automatically.

Waterloo up platform is on the lower right. The platforms are staggered so the down platform is behind the camera.

Stop Signal

The Ganz Mavag EMU has passed 80 Down Starting From Down Main (Waterloo) which has changed from Green over Red to Red over Red.

Signal 74 Down Directing (Woburn) in the distance is showing Green over Red.

80 is a Stop & Stay signal converted to Stop & Proceed by the illuminated "A" light.

Stop Signal- Two sections occupied

The front of the EMU has passed 74 Down Directing (Woburn). and so 74 has changed to Red over Red.

Stop Signal - Overlap Track Circuit Occupied

The rear of the EMU has passed signal 74 so the section between signals 80 and 74 is now unoccupied. However, signal 80 is still showing Red over Red because the overlap track circuit in advance of signal 74 is occupied.

A better view of the overlap is in another photo at the end of this set.

Caution, Normal Speed Signal

The EMU has cleared the overlap track circuit in advance of 74 so signal 80 has cleared to Yellow over Red. The tail lights of the EMU, which is stopped at Woburn station, can just be seen.

Clear, Normal Speed Signal

The EMU has passed signal 68, which is the signal next in advance of 74, and has also cleared the overlap track circuit. Signal 74 has cleared to Yellow over Red and 80 has cleared to Green over Red.

A better photo of the Overlap.

This photograph is from another set. I have included it because the overlap is more obvious.

Absolute Block

Although not relevant in the New Zealand context it is interesting to note that overlaps were an integral part of British Absolute Block (manual) signalling. In particular, Absolute Block Regulation 4 (I am quoting from a book published in the 1940s) stated that a signalman could signal Line Clear only when the line was clear for 1/4 mile ahead of the Home signal and all points necessary for the approaching train had been set in the proper position.

At junctions the line was required to be clear for 1/4 mile ahead of the junction points and all points properly set for the approaching train. The conditions under which Line Clear could be given for trains approaching a junction could become quite complicated, depending on the position of the points, the location of other trains and whether or not there were Outer Home signals.

Last Updated: Saturday, December 31, 2005

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