Part 13 (2/2)

ELECTRIC TRAIN STAFF AND TABLET SYSTEMS

These syste are developreater safety and ease in working the line On sole line circu of several trains in one direction without a return train For such cases the train staff ticket was introduced; but even on the best regulated lines it is not always possible to secure that the staff shall be at the station where it is required at the right ti available at the station where the staff was, it had to be taken to the other station by amuch delay to traffic The electric train staff and tablet systems overcome this difficulty Both work on much the same principle, and ill therefore describe the former

[Illustration: FIG 101--An electric train staff holder: S S, staffs in the slot of the instru the key K at the end for unlocking a siding points between two stations The engine driver cannot reain]

At each end of a block section a train staff instru 101) is provided In the base of these instruments are a number of train staffs, any one of which would be accepted by an engine-driver as perle line The instru that a staff can be withdrawn only by the co-operation of the signalman at each end of the section; that, when _all_ the staffs are in the instruments, a staff may be withdrawn at _either_ end; that, when a staff has been withdrawn, another cannot be obtained until the one out has been restored to one or other of the instruments The safety of such a syste by having a staff available for a train no matter from which end it is to enter the section

The mechanism of the instruized by the depression of a key by the signalman at the further end of the block into which the train is to run, and by the turning of a handle by the signalized, is able to lift a mechanical lock, and perh the instrument the staff revolves a number of iron discs, which in turn raise or lower a switch controlling the electrical connections This causes the electric currents actuating the electro-netis another staff being withdrawn It will naturally be asked, ”How is the electrical system restored?” We have said that there were a nuiven nuned to the section assume that there are ten in each instrument, and that the switch in each is in its lower position Noithdraw a staff, and one instrument has an odd, the other an even, number of staffs, and similarly one switch is raised while the other remains lowered, therefore the electrical circuit is ”out of phase”--that is, the currents in the nets of each staff instrument are opposed to one another, and cannot release the lock The staff travels through the section and is placed in the instru the number of staffs to eleven--an odd nu_ the switch Both switches are now raised, consequently the electric currents will support each other, so that a staff may be withdrawn Briefly, then, when there is an odd number of staffs in one instrument and an even nunalive authority for a train to enter the section; but when there is either an odd or an even number of staffs in each instrument a staff nalmen

We nal-box, one for working to the box in advance, the other to the rear, it is arranged that the staffs pertaining to one section shall not fit the instrument for the other, and must be of different colours This prevents the driver accidentally accepting a staff belonging to one section as authority to travel over the other

INTERLOCKING

The renals on double lines apply also to the working of single lines, with the addition that not only are the distant, honals interlocked with each other, but with the signals and points governing the approach of a train fronals for the approach of a train to a station from one direction cannot be lowered unless those for the approach to the station of a train froer, and the points correctly set

SIGNALLING OPERATIONS

In the working of single lines, as of double, the signalman at the station from which a train is to proceed has to obtain the consent of the signal very similar to those detailed for double lines There is, however, one notable exception On long lengths of single line it is necessary to ements for trains to pass each other This is done by providing loop lines at intervals, a second pair of rails being laid for the accommodation of one train while another in the opposite direction passes it To secure that le line between two crossing-places it is laid down that, when a signal station is asked to allow a train to approach his station, he nal permission for trains to approach frole and double line working a nuency are laid down, the guiding principle being safety; but we have now dealt with all the conditions of everyday working, and must pass to the consideration of

[Illustration: FIG 102--An electric lever-fra cabin at Didcot]

”POWER” SIGNALLING

In a power systenalman is provided with some auxiliary nals or points under his control It is still necessary to have a locking-franal-box, with levers interlocked with each other, and connections between the box and the various points and signals But the frame is much smaller than an ordinary manual frame, and but little force is needed to move the little levers which make or break an electric circuit, or open an air-valve, according to the power-agent used

ELECTRIC SIGNALLING

Fig 102 represents the locking-fraland, where an all-electric system has been installed Wires lead fronals, which they operate through wor When a lever is h a motor, the direction of the flow (and consequently of theon whether the lever has been ed under the levers tell the signalnals have been co to failure of the current or obstruction of the working parts, an electric lock prevents hi operations Thus, suppose he has to open the ed by the -franals can be lowered He ht move all the necessary levers in due order, yet one set of points nals lowered, an accident would result But this cannot happen, as the electric locks worked by the points in question block the signal levers, and until the failure has been set right, the signals er”

The point nalof a powerful electro-nal it is necessary that the net to give it the requisite attractive force

If no control current flows, as would happen were any pair of points not in their proper position, theto the nal had been already lowered when the control current failed, it would rise to ”danger” autoer position by gravity The signal control currents can be broken by the signalnals er

PNEUMATIC SIGNALLING

In England and the United States conalnal a lever half-way over, ad the track to an air reservoir placed beside the points or signal to which the lever relates The air opens a valve and puts the reservoir in connection with a piston operating the points or signal-ar been performed, another valve in the reservoir is opened, and air passes back through a second pipe to the signal-box, where it opens a third valve controlling a piston which conalman that the operation is complete With co-frame is of course used

AUTOMATIC SIGNALLING

To reduce expense, and increase the running speed on lines where the sections are short, the train is sonalether so as to be in metallic contact, and each section is insulated fro sections At the further end of a section is installed an electric battery, connected to the rails, which lead the current back to a nal stationed so as current flows the signal is held at ”All right” When a train enters the section the wheels and axles short-circuit the current, so that it does not reach the signal er,” and stays there until the last pair of wheels has passed out of the section Should the current fail or a vehicle break loose and re would happen

The hu To s absolutely safe, a train should have positive control over a train following, to prevent the driver overrunning the signals On electric railways this has been effected by nals, which either cut the current off fro that on which a train er to strike an ar and apply its brakes