[Fig. 513] is a diagram sketch showing the pair of instruments as they stand on the instrument-tables in the signal-cabins B and C, where B2 and C1 are the instruments which work together for the block section BC. Supposing a DOWN train proceeding from A in the direction of F, and approaching the signal-cabin of the block station at B, the DOWN starting signal standing at danger; then by the code of signals on the bell or gong the signalman at cabin B would communicate with the signalman
at cabin C, to obtain line clear, so as to allow the approaching train to proceed on to C. If the previous train in the same direction had already passed C, and there was not any obstruction on the line, the signalman at C would give line clear for the DOWN train, and to do so he would turn his circular disc to show the WHITE inscription ALL CLEAR, and then push in the plunger of his C1 instrument, lowering the DOWN or white arm, K, of his own instrument to the position shown by the dotted lines, which operation would at the same moment lower by electricity the DOWN or red arm, G, of the instrument B2 in cabin B to the position of the dotted lines. The signalman at B would then lower his starting signal, to allow the DOWN train to proceed on towards C, and immediately the train had passed the starting signal he would, by means of his bell or gong advise the signalman at C that the train had entered the section, or block BC, and the signalman at C would at once turn his circular disc to show the RED inscription TRAIN ON LINE, and use his plunger to raise to danger the DOWN or white arm, K, of his own instrument, and at the same time raise by electricity the DOWN or red arm, G, to danger in the instrument B2 in cabin B. The section BC would then remain blocked until the DOWN train had arrived, or passed the station C, when the signalman there would, by means of his bell or gong advise the signalman at B that the DOWN train had passed out of the section, and would turn his circular disc to show the GREEN inscription TRAIN OFF, BUT SECTION BLOCKED. Both instruments would then be in their normal positions, with the arms raised to danger, and ready for further train operations. In a similar manner for the UP-line trains on the section or block between C and B, the signalman in B cabin would turn his circular disc, and use his plunger to lower the UP or white arm, H, in his own instrument, B2, and at the same moment lower by electricity the UP or red arm, I, of the instrument C1 in cabin C, the other operation for train on line and train off being carried out for the UP train in the same routine as for the DOWN train. The outdoor fixed signals, or distant home and starting semaphore signals, have all to be worked to correspond to the block telegraph signals, and as the latter are always received well in advance of an approaching train, it follows that when the line is clear, the outdoor signals can be lowered so as to allow a through or non-stopping train to pass a block-telegraph station at full speed.
Where the traffic is moderate, it may be sufficient to have block-telegraph instruments at each of the stations, but with a very frequent train service it will be found necessary to divide the line into shorter sections, and erect signal-cabins and block-telegraph instruments at intermediate points between stations.
The code of bell or gong signals is extended to include various matters in connection with the train-working. For example, when a DOWN train is passing cabin B at full speed, the signalman may observe that there is something wrong—a carriage or waggon on fire, a tail-lamp missing, or other irregularity. It is too late to stop the train with his own signals, but by means of his bell or gong he can call upon the signalman in cabin C to stop and examine the train, and the DOWN distant and home signals at C can be raised to danger before the train reaches the cabin at C.
In every block-telegraph signal-cabin there is a train-book in which the signalman has to write down the time and description of every arriving or passing train, and, as this book lies before him, he has a complete record of the train-working, with the particulars of the exact times when the line clear signals were given, and also when the train arrived or passed his signal-cabin.
To guard against the possibility of a signalman inadvertently giving line clear, or allowing another train to pass his cabin before the previous train had reached the signal-cabin in advance, some railways have adopted the lock and block system. By this arrangement the starting signal at any cabin is electrically and mechanically locked from the cabin in advance, and can only be released or lowered by the action of the outgoing train itself when passing over a treadle or other appliance connected with the rails of the running-line at the signal-cabin in advance. This method practically gives the train the complete control of the section; and any signalman attempting, in error, to lower his starting signal would find it to remain fixed to danger and immovable, until released by the arrival of the train at the advance cabin.
Train-staff for Single Line.—When there is only a single line of railway for both an UP and DOWN train-service, very definite precautions must be adopted to prevent the meeting or collision of trains travelling in opposite directions. Where the piece of
single line is short, and can be worked by one engine in steam, or two coupled together, no collision can take place, as the train-service will be limited to the one train moving backwards and forwards over the section; but with a long length of single line, including a large number of stations, necessitating several trains, some clear and comprehensive regulations must be introduced. For a long time the simple train-staff was found to give the desired security; there was only one staff for each pair of adjoining staff-stations, and no train was authorized to run without the staff, and as the staff could only be on one train at a time, the precaution against collisions was looked upon as complete. These staffs, which were generally made of brass, or other metal, were sufficiently large to be conspicuous when placed in the stand prepared for them on the engine. They were lettered to correspond to the stations to which they belonged, and were made in different patterns to distinguish them for their respective sections. No train was allowed to start from a station until the engine-driver received from the station-master the proper staff to authorize him to proceed to the next station, and on his arrival there it was the duty of the engine-driver to hand over the train-staff to the stationmaster of that place, and wait for another train-staff to authorize him to proceed over the next section. So long as the train service could be evenly arranged, and that there was always an UP train to take back a train-staff which has been carried out by a DOWN train, the simple staff worked most efficiently; but as the traffic increased, and two or more trains had to be despatched in the DOWN direction before one had to run in the UP direction, some auxiliary arrangement had to be introduced. This was effected by issuing train tickets, kept in a locked-up box, which could only be opened by the key attached to the train-staff. A properly dated train-ticket was handed to the engine-driver of the first DOWN train, and, if necessary, a second train-ticket to the engine-driver of a second DOWN train, and then the train-staff itself was handed to the engine-driver of the third DOWN train. There were one or two serious drawbacks to this train-staff and ticket-working. As there was only a time interval between the starting of the trains, the one train might overtake and run into the other with disastrous results. Again, a second or third train, which was put down in the schedule, might be withdrawn at the last moment, and the staff left behind at a station when it
was required at the opposite end of the section, thus causing much confusion and delay. The ordinary electric telegraph could have been utilized to assist in regulating these train movements, but it was felt that a mere telegraph message was not sufficient to ensure positive safety, and that something more tangible was required in the shape of a staff, or token, without which no train should be allowed to travel on a single line of railway. To meet this requirement, the electric train-tablet, and the electric train-staff instruments have been invented, each of them being so arranged that upon any one section, or pair of instruments, a tablet or train-staff may be taken out from the instrument at either end of the section, but when once taken out, no other tablet or train-staff can be withdrawn from either instrument until the first has been delivered and placed again in one or other of the two instruments.