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The New Zealand Railways Magazine, Volume 1, Issue 10 (March 21, 1927)

The Insulated Track

The Insulated Track.

This consists of the permanent way bonded together at each joint and divided at suitable intervals into sections by insulated joints on each rail of the track. These sections are thus insulated from each other, and the sleepers and ballast are of sufficiently high resistance to insulate the rails from the ground; as very low voltage is used for track electrification (below 5 volts) and leakage to ground is negligible at low pressure owing to the size of metal in the rails providing the easier path for the current. The current fed into each section of track flows up one rail and down the other within its own section, and this current operates a relay or relays situated at the end or both ends of this section of track. Where a track carries a relay at each end as on single line working the current is fed to the centre of the track and flows both ways to the ends of the section returning on the other rail to the centre.

Three Position Colour Light Signals. Method of showing Clear, Caution or Danger through insulated track and three-position relay.

Three Position Colour Light Signals.
Method of showing Clear, Caution or Danger through insulated track and three-position relay.

Relays are electrical devices for opening or closing circuits. For track work they consist roughly of two kinds: those which operate two sets of contacts (switches)—known as two position—and those which operate three sets of contacts (switches)—known as three position. The two-position relays close one set of contacts (switches) when current is flowing through their operating mechanism and open that set and close another set when current is cut off.

The three position relays are differently operated as they open and close two sets of contacts (switches) according as the current flows through their operating mechanism from right to left or from left to right. They also open both these sets of contacts (switches) and close a third set if the current is cut off by a vehicle being on the track or other cause.

It will be seen from this that the insulated track can provide, through its relay, for three-different operations, that is, three circuits can be switched in by the relay, one to light a green light, or one to light a yellow light or another for a red light according to the position of the contacts (switches) provided in the relay.

(To be continued.)