197503 – Burton – Signalling Developments on the Victorian Railways at Oakleigh: Rail Clamp Point Lock Machines

We are all well aware that the aim of point locking is to ensure that the point blade
is held firmly and locked against the stockrail during the passage of a train and that this
locking can be electrically detected in the protecting home signals.

From an earlier paper on point locking by Mr. 0.S. Nock in 1959 it is shown that all early point locking was done at the stock rail and point blade, but as track maintenance improved and the speed of trains increased it was decided to use plunger locking and this was located either between the point blades or in the 6’0″. This type of locking has continued with the introduction of pneumatic and electric point mechanisms.

On all types of interlocked points in service on the Victorian Railway System, the locking has taken place using plunger locking.

Initially with mechanical operation of points the locking was done within the track using a plunger, plunger box and stretcher bar. On later designs this locking was relocated outside the track using a plunger box and if the points are operated electrically or pneumatically the locking is incorporated within the machines.

These methods of locking require relatively close tolerances to be maintained in the lock box and detection and are in turn subject to drift out of adjustment if there is any relative movement between trackwork and supporting timbers.

In Victoria the point mechanisms are not directly tied to the stock rail and as timbers deteriorate the trackwork can move independently of the mechanism and plunger boxes causing out of adjustment failures.

From these opening remarks it can be seen the wheel has taken a full turn and today we are looking at devices that positively lock the stock rail and point blade together.