Some of the posts referred to below are still in preparation.
Mike's Railway History includes a brief description of Burma's railways in the 1930s which appears to be from one of the contemporary railway magazines. One paragraph describes signalling:-
The standard interlocking used for all single-line crossing stations on important lines is known as the Simplex system, while there are also a few controlled by List and Morse interlocking. In the Rangoon area the lock and block system is installed on the double line, and about forty double-line stations are interlocked on the key and tappet system. Some twenty junctions and important stations have fully-interlocked cabin locking. On most branch lines the stations are not interlocked, facing points being secured by cotters and padlocks, the keys of which are kept in the custody of the station-master on duty.Another paragraph describes the principal station in Rangoon (now Yangon). At the time, this station was called 'Phayre Street':-
The passenger station yard is controlled by three principal signal boxes, two of which have about a hundred levers each, and the third about seventy-five The Western Electric system of train control is installed throughout the Rangoon area and interlocking is very complete. All signal lamps are electrically lit.I haven't traced other references to 'Simplex' interlocking but 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) seems similar.
Single-line crossing stations
The Indian system for controlling simple passing loops has a Station Master (of high integrity) in charge, with two Pointsmen who can be despatched to hand operated points at each end of the loop 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 'Trapped Key' method has two keys for each set of points which can be inserted in a Locking Box between the rails at the toe of the points. A point stretcher passes through the locking box arranged so that if the points are correctly fitted-up in the 'Main' position, the 'Main' key can be extracted from the Locking Box. Actually removing the key locks the points in that position. The other key remains 'trapped' in the Locking Box. Once the 'Main' key is replaced in the Locking Box, the points may be operated.
Similarly, with both keys present, if the points are operated to the 'Loop' position and are correctly fitted-up, the 'Loop' key can be extracted from the Locking Box and, once removed, locks the points in that position. Now, the 'Main' key remains 'trapped' in the Locking Box.
Thus, if the 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. If the Station Master receives two 'Loop' keys, a train may be passed over the loop line. In India, a further development allowed splitting home signals to be worked from a ground frame on the platform.
The arrangements I've seen so far in Burma seem to conform to the above description, except that the splitting home signals are locally controlled by the pointsmen with a further 'Trapped Key' locking box being provided in the push rod to the signal arm (see Part 4 - Manual Control of Points and Interlocking for more information).
The picture below shows a partly-dismantled locking box no longer in use and is included to clarify the construction.
A single-lever ground frame and point indicator on the Circle Line at Yangon. The points are provided with a 'Trapped Key' Locking Box which has been partly dismantled but the points have been clipped and locked on the nearer rail to make them safe.
Click here for larger version of the above picture.
'Trapped Key' Interlocking is still widely used in industrial safety in the form of Castell Interlocks (introduced by James Harry Castell in 1922 adapting ideas first used on the railways).
Double-line control
The Circle Line in Yangon is double track, as is the line from Mahlwagon to Bago. Other parts of the system are also double-track (and doubling is being extended) but, at present, I've no information on the original arrangements.
The 1930s description quoted above says "The Western Electric system of train control is installed throughout the Rangoon area". This system originated in North America as an electro-mechanical secret party-line selective telephone system where a number of waystation telephones were all connected to the same pair of wires. A series of impulses sent to line from the control station normally selected only one telephone to speak to or hear the control. This secrecy made it a safe method for communicating train movement instructions as they could not be overheard (and misunderstood) by other waystations. In addition to Western Electric exports, the system was licensed by Standard Telephones and Cables in England and they not only exported systems (for instance, to Thailand) but also adapted the system for use as a Signal Post Telephone system in the U.K. For more information on this system, my friend Sam Hallas has produced a comprehensive article Control Telephone Systems.
Any form of telephone system needs telephone wires between the various instruments. Originally, this would be open wires on a system of poles but latterly multicore cables are used. See 'Part 7 - Telecommunications' for more information.
Regarding Rangoon, the 1930s description quoted above reports "The passenger station yard is controlled by three principal signal boxes". In 1946, an order was placed with Westinghouse Brake and Signal in Chippenham for two Style 'L' miniature lever frames. One, intended for Rangoon (now Yangon) had 143 levers, the other, for Kemmendine (now Kyee Myin Daing) had 47 levers. All signals in the Yangon area are currently colour light with power operation of points but, as yet, I've not determined whether the Westinghouse lever frames remain in use. See the post Yangon Area Railway Signalling for more information.
The 1930s description quoted above reports "About forty double-line stations are interlocked on the key and tappet system". This is presumably a variant of the 'Trapped Key' system where keys available physically control which signals can be operated. This may be similar to the method of signal release I've found in Burma (see Part 4 - Manual Control of Points and Interlocking for more information).
The 1930s description quoted above reports "Some twenty junctions and important stations have fully-interlocked cabin locking". Bago North and Bago South signal boxes are certainly 'fully interlocked'. See the post Railway Signalling in Burma: Part 5 - Signal Boxes with Interlocking Frames.
References
Mike's Railway History
Burma's Railway System.
(This PDF appears on the website of the Old Martinians Association U.K. forwarded by Peter R. Moore. Some of the text also appears on the Mike's Railway History Page but the Peter Moore article includes additional material and a number of more modern photographs showing steam locomotives).
Control Telephone Systems.
[Photograph of part-dismantled locking box added 20-Mar-2013]
