Interlocking A Station.

Having decided, on the grounds indicated above, whether a mechanical, all electric, or partial electric system is most advisable at any new installation to be undertaken, the interlocking expert proceeds as follows:

The various movements and combinations of movements required for the most efficient working of the yard are determined by ascertaining the requirements of the Traffic and Locomotive branches, together with particulars of train lengths to be handled and the density of traffic to be provided for. An eye must also be kept on probable extension of the interlocked area and projected alterations in the near future. This information is co-ordinated with certain practice derived from the experience of other countries, modified as required to suit New Zealand conditions.

The required signals are then placed on the yard diagram, and their levers in the interlocking machine allotted. The points and trap points in the interlocked area, with the facing point locks (if mechanical installation) are also given lever numbers, thus deciding the size of the machine required.

The locking plate is then designed, in terms of lever numbers showing all possible combinations of levers that can be pulled together with safety and yet fulfil the requirements stated above.