Railway Signalling in Burma - Part 4: Manual Control of Points and Interlocking

In the previous article on railway signalling in Burma Part 3 - Control of Trains, I reviewed my understanding of the origins of the systems in use. But I started to work out the methods in use much earlier, from observations I made from the cab of a diesel locomotive described in the post Cab Ride from Katha (and the associated posts and links to pictures).

Single Line Passing Loops

At passing loops, the only signals are normally a splitting home signal at each end of the station, situated at the toe of the points, allowing movements onto either Main or Loop line. The signal arms may be arranged as upper-quadrant or lower quadrant but both types are operated by push rods from the ground, interlocked with the loop points. Normally, a single lever ground frame operates the signal with the appropriate arm 'selected' by an interlock with the points. There are also various 'EIC' lock boxes in between the rails at points and mounted near the base of the signals. Apparently, some areas have elevated, multi-lever ground frames by MacKenzie and Holland but stations and junctions I've seen to date use a single-lever ground frame adjacent to the points, provided with a (usually rather derelict) point indicator.

'Trapped Key' Interlocking

The 1930s article refers to 'Simplex' interlocking which I haven't traced but I assume refers to the 'EIC' equipment. I haven't identified 'EIC' either so if anyone has any information it would be appreciated. By studying my photographs, I'd largely decided how the interlocking was intended to work but then I found a Paper by R. C. Rose titled 'A Survey of Indian Signalling' in the 1924 Proceedings of the Institution of Railway Signal Engineers (available online here) which seemed to confirm the philosophy. 'Trapped Key' Interlocking is still widespread in industrial safety as Castell Interlocks (introduced by James Harry Castell in 1922 adapting ideas first used on the railways).

Consider the simplest form of the railway passing loop. A single line splits into two through the station and then the two lines re-converge into the ongoing single line. There are just two sets of points. To reduce cost, both sets of points are operated local to the switch by hand. In Burma, a Single Lever Ground Frame is provided adjacent to the toe of the points and a Point Indicator may be fitted.

Ywa Taung: Single Lever Ground Frame which operates the points via the 'L' crank and Drive Rod on the right. Note that the long arm of the 'L' crank is adjustable, allowing the 'throw' to be adjusted. The Point Indicator on the left is operated directly from the points via a separate delector rod.

The Station Master has two Pointsmen who can be despatched to the handpoints to set them as required. But if the pointsmen make errors, trains can arrive on the wrong track, departing trains can 'trail' incorrectly set points or points can be changed as a train is passing over them.

So, various forms of key-locking apparatus were produced with the aim of giving the Station Master confidence that his instructions had been correctly carried out. The EIC Locking Box is one method of establishing this confidence, using the 'Trapped Key' method.

Detail of EIC Locking Box fitted between the rails on the Loop Points at the south end of Naba station. The Loop Handles on the two lock slides are at opposite ends of the box. The interlocking key is clearly visible on the left of the box. (In addition, the points have been clipped in the normal position - the point clip is on the left hand stock rail).

View of another Point Locking Box fitted between the rails (this one at Ywa Taung). Note the slot on the right of box for the interlocking key. There are two lock slides with loop handles. Each switch rail appears to have a separate detector blade.

What I've called the 'Lock Slides', manually moved in and out using the loop handles, serve the function of locking the points normal or reverse. In normal British Practice, there is one 'Lock' which can pass through either of two notches in the Point Stretcher. But in the EIC Locking Box, there is one notch, allowing one 'Lock Slide' to pass through the notch when the points are set to 'Main' and the other 'Lock Slide' to pass through the notch when the points are set to 'Loop'. As a further refinement, on some designs, each switch rail has its own notched detector blade so that both switch rails are separately detected.

If the 'Main' 'Lock Slide' can be pushed in, the 'Main' Key in the side of the Lock Box can be extracted. Once the Key is removed, the 'Lock Slide' cannot be pulled out and the points are held in the 'Main' position until the 'Main' Key is re-inserted. In a similar way, the 'Loop' Key can only be removed if the points are proved in the 'Loop' position.

Ignoring the complication of signals, if two Pointsmen each present 'Main' Keys to the Station Master, he may be confident that the points are set for the 'Main' at both ends of the loop.

In India, a further development allowed splitting home signals to be worked from a ground frame on the platform. In Burma I've not (yet) seen such an arrangement but at many passing loops splitting home signals are provided but worked locally.

Interlocking between Loop Points and Push Rods for selecting the signal arms of the Down Inner Home at Naba. Note the two 'EIC' lock boxes.

My Pictures

Burma: Control of Points and Interlocking.

The above set of pictures is part of a Collection Railways in Burma.