Mechanical Signal Cabins

By Ravindra Bhalerao, 2017

MANY YEARS AGO an alarming news report appeared in one of the dailies here, titled "Railway signal cabins will soon become history." The feature described what was happening in Central India, as indeed all over the country, with the railways relentlessly pursuing a drive to adopt a modernized signalling system. Centrally operated signalling had been commissioned in Jabalpur division. And with it the age of cabin signalling, ever present on the railways since earliest times, was fast drawing to a close. Nearabout 130 conventional cabins in the Jabalpur area were made redundant following the installation of the new system; the 500 or so men working in these cabins had to be consequently redeployed elsewhere.

Signal Cabin A

The newsreport sent a wave of panic surging through me, though not strong enough to make me want to go out and collect data on the cabins that still remained in operation. I remained calm, basking in the memory of those good old days when I would peer out of the window of the train in the dark, finding the city lights sliding past, the train going a mighty clang-clash over the points and crossings, the sound heightened by its echo against a nearby wall -- ah, my train was passing the cabin, and pressing my forehead against the window bars I would catch a glimpse of the lone man high up in the window holding a feeble green light signalling us on to the station that lay ahead.

Semaphore signal

Conventional mechanical signalling has been in use for over a century on our railways and may still be found over isolated sections of the network. In mechanical signalling, both signals and points are actuated by manual power, and to save time and space, the controlling levers are housed together in a row in an elevated structure known as a signal cabin. The interior of a cabin will be found to contain an impressive array of levers; some of these are for operating points, others for signals, whilst still others are spare levers to be called into use should the need arise when the yard is remodeled or expanded.

Signal and point levers actuate rods which are below the floor lever of the cabin, and these are interlinked together in an elegant way using locks and tappets so that, for instance, it is impossible to lower a signal for an approaching train unless the points that relate to the route are properly set and locked. In a similar manner, interlocking, as it is known, also ensures that it is impossible to lower signals for the admission of trains from opposite directions to the same line at the same time.

Interlocking thus imposes a constraint on the signalman; it is a refinement whose purpose is to ensure safety in train movements. The greatest source of danger in train operation, however, arises when two trains are dispatched along the same line, one following the other. This is a problem railwaymen have grappled with since earliest times; it was only after the invention of the electric telegraph that a successful way could be devised to deal with the trouble, set forth briefly below.

What can be expected when two trains are dispatched one following the other on the same line? When the first commercial railways were opened, the need for signalling was not urgent. Speeds were low and in the face of an obstruction, a train could be easily brought to a quick halt. Policemen were often employed at intervals along the track to signal to the driver if any such obstruction was spotted.

As technology progressed, it became possible to operate longer and therefore heavier trains. As speeds of around 50 kmph were reached, a train often needed half a kilometer or so of braking distance to pull up. Accidents such as a train ramming into another a short distance ahead were common enough.

The first step towards improving the safety of rail travel was the adoption of a "time interval" system of operation. By this it was meant that if two trains were awaiting departure at a station, the second one to leave would be signalled to leave only after, say, 10 minutes had elapsed after the first one had departed. By this method it was hoped that there would be a safe distance, equal to 10 minutes of running time, between the two trains.

This method, although admirable in theory, did not always work the way it was intended to. If the first train halted midway due to a breakdown (and this was a common enough occurrence in those days), there was no way that the driver of the second train could know that he was heading towards an obstruction. To avoid a collision, the guard of the first train was supposed to run backwards along the track to attract the attention of the oncoming train by gesticulating, even shouting.

The modern era in railway signalling was ushered in only after the arrival of the electric telegraph. The fundamental merit of the telegraph and other similar devices is that it makes possible instantaneous communication between two points. Telegraphic apparatus installed at stations made it possible for one station to communicate with the other regarding the arrival and departure of trains using morse code.

The advantages of this means of communication were soon realized. If one train were to be signalled for departure, the fact that it had pulled out was telegraphed to the next station ahead. And on arrival there, the signalman there telegraphed a message back saying that the train had arrived intact. Till such time as this return message was received, no other train would be allowed to proceed along the same line, thus eradicating the possibility of a train mowing into another that had broken down midway along the line.

This simple method, in an improved form, has become the basis of what is known as the "Absolute Block System". In the Block System, a railway is split up into a number of short sections, known as block sections, each section generally being the stretch of track from one station to the next one ahead. Once a train has pulled out and enters the block section, this fact is intimated to the station master ahead. This gentleman ahead holds "charge" of the block section, as it were. While the train is on its way, this gentleman knows that the block section under his charge is occupied. During this period, therefore, if the signalman in rear were to ask his permission to dispatch a second train, he would get a blank no. Once the train has arrived at the station ahead, our station master knows by looking at the tail lamp that the whole of the train has arrived intact. His block section is now clear, and he is in a position now to give permission to approach (or give line clear, as it is known) to a train awaiting departure at the previous station.

The working of trains on the absolute block system clearly involves a steady stream of messages between signalmen along the line. At first these messages were dispatched using the electric telegraph. In due time, "block instruments" were devised to perform the same function in a far more convenient manner, avoiding a whole lot of tiresome telegraphy. This ingeniously devised instrument often is a nightmare for the casual observer ; it is in fact simply an electomechanical device installed in cabins to assist the station master by providing visible indications regarding the status of his line. For example, the instrument may have a needle pointing to line clear, line closed, or train on line etc. Block instruments also carry bells by which a signalman can communicate with his colleague down the line using a bell code.

Here is a picture of a smart young Cabin ASM (seated) manning a signal cabin. To the right of his assistant can be just seen the double-line 3-position block instrument painted in green:


Single line stretches of track, too, are controlled by block instruments, but of a different kind. The single-line token instrument pictured below releases a hollow metal ball, known as the ball-token, that is handed over to the driver of a train as the authority to proceed.

Single Token Instrument

Close to the row of levers which actuate the points and signals, the cabin has a shelf for housing these vitally important block instruments and associated electrical paraphernalia. Signal cabins are also required to have a clock updated each day according to radio time, telephone sets, a large overhead diagram showing the track layout of the station, a copy of the signalling rule book, various record keeping registers, a chart listing the various forms and orders the signalman is likely to deal with, a set of red and green lanterns and flags, and a supply of kerosene.

At large stations, a cabin is manned by a person of the rank of Assistant Station Master with one or more levermen working under him. During his shift of duty, the cabin ASM and his assistant form a vital part of the machinery that controls the movements of trains, and on their alertness and devotion depend the safety of hundreds of passengers each day. The Cabin ASM is fully trained for his job. He has to be thoroughly familiar with the railway rules for working trains and the layout of the yard over which he holds charge. Besides operating the block instruments on the shelf, he has to keep a constant watch for any sign of danger on running trains, and is in constant touch with the control office. His block instrument has freed him from a lot of burdensome telegraphy, but this does not leave him free of paperwork. His desk will have a ready supply of blank order forms to be used when the need arises. His work in connection with the passage of trains requires him to constantly make entries in a voluminous book known as the train register.

Cabin Lamp
Train Register

Is the signalman's job an enviable one? It depends on what you mean by the word signalman. A cabin ASM sure will be having great fun, some people think, but we must ever bear in mind that if an accident ever takes place due to signalling errors, it is the CASM who is hauled up for negligence. Levermen, on the other hand, are relatively free of responsibility, for they have merely to pull the levers. Dr. Ardeshir Damania has given us an excellent insight into the working conditions of the leverman. Dr. Damania had the good fortune to actually see for himself how the leverman worked while in Navsari a long time ago, and his recollections of the time are as clear today as though it all happened yesterday.

"As a young boy, visiting my aunt at our ancestral home in Navsari, I with my cousin climbed up into the Navsari main signal cabin," recounts the doctor. "The telephone rang and some light went up or off in the cabin indicating that a train was about to arrive. The signal man then braced himself by putting his right leg at the base of a long lever and depressed a smaller handle at the top which was a kind of a locking lever). Then he began to strain with all the strength in his body to pull the lever, stopping several times to catch his breath, until finally the handle came down and the outer signal blade turned from red to green! He then told me that outer signals were almost 1/4 mile away and hence they needed the most strength. It was not a job for the weak! Due to counter weights on the signal blade itself, it was not that strenous to move the long lever from green to red after the engine had crossed the signal. The long lever went back to its upright position with a big "clang". When all the levers were upright, it meant that both up and down signals were in red position and the level-crossing could be opened for traffic."

Dr. Damania is has always interesting details to share with his readers. "I forgot to add for our younger readers," he goes on to tell us, "that the wooden signals on top of an iron lattice base/pillar were connected to the signal cabin my a steel wire about 1/4 inch in diameter and hence the strength needed to operate the signal. The incident at Navsari that I have narrated took place around 1954. They had both metal and wooden signal blades. The metal ones, except those made from galvanized steel would rust. Both wooden and metal ones would have to painted at least once a year after the rains. By the way, the same levers and handles were also connected by point rodding to change the tracks. The amount of strength of one man needed to operate these were in direct proportion to how far from the cabin the mechanically operated signal or the track changing mechanism was. If it was too far away then a counter balancing iron weight was put midway so that the weight made it easy for operating the signal or the track changer both ways. With the arrival of electronic signals these mechanical signals disappeared one by one and signal cabins became a thing of the past. Electronic track changers made the life of the cabin-man extremely easy."

Signal cabins of old always evoke fond memories. "I remember the signal cabins at Ballygunge Station (a suburban station) near our house in Calcutta, where my grandfather often used to take me," says Dr Tathagata Chatterji, who was formerly Editor of the Indian Steam Railways Magazine, and a keen railway heritage enthusiast himself. "Taking an evening walk along the track and then chatting with station staff was his daily routine. But I once had a very interesting experience. I was visiting Jharsuguda in Orissa, with my grandfather. The station master was his college friend. Jharsuguda, those days was a very sleepy place. Elephant herds from the nearby forest used to cross the track. One evening while I was sitting at the cabin, the signalman drew my attention to an elephant whose leg had got trapped in the track. It was a truly memorable scene and I saw how other animals in the herd pulled and pushed to rescue their mate. Fortunately no trains were due then, and no accident happened. But it left me thinking if one's leg got trapped that way, what would have happened. . . "

Signal cabins make a fascinating subject for study. With increasing centralization of control, many of these cabins, particularly those in goods yards, are falling into disuse. It is sad to think that no one has earmarked these lovely old structures to be preserved as objects of heritage value, objects that, like locomotives and station buildings, stand as true symbols of India's colonial railways.

Once a cabin has become redundant, it is labeled "abandoned," a very apt description, for the cabin is left to its fate to bake, rot and rust. Only last week I was driving through town when I spotted one such old cabin. With its sloping tin roof, it looks like a snug little railway cottage. Here is a picture for the record:

Cabin side

There is nothing in the picture above to suggest that this structure is, in fact, a railway cabin, so I took another shot the next day from atop the overbridge:

Cabin top

Walking over to the other end of the overbridge presented a view in full sunshine:

Cabin - abandoned

Note the "abandoned" printed in bold letters. The sloping roof bespeaks of British style architecture, but I have not researched the subject yet, and hence can not specify the actual year of construction.

Do I get the whiff of engine smoke as I write these words? John Masters did when wrote his Bhowani Junction : "From the windows of Bhowani North Box you can see all the lines and over the platform canopy into the yards. The row of big shiny steel levers stretches along under the front windows, and the block instruments are just above the windows. The lights move on the track circuiting diagram whenever a train moves about within the station limits, and the telegraph bells ringding - ding, and you can hear the clank of buffers and smell the engine smoke..."

Last cabin