The block system of running railroad trains has been generally adopted in England. In this country it has not yet come into very extensive use. As hitherto installed, it requires the presence of an operative at the termination of each block. The theory of its operation is simple. It consists in the use of danger signals at the beginning of specified lengths or blocks of track, each of which signals is to be kept at danger when a train is on the block protected by it until the block in question is free. In practice a block may be from 1,000 feet to three miles in length. To facilitate traffic to the highest degree, the length should not exceed one-half mile. To introduce the block system, therefore, upon a line of road under the old system is very expensive, owing to the number of men required to run it. To reduce this expense there is a constant temptation to increase the length of the blocks, thus making it less efficient. A system which depends upon human vigilance can never be considered a perfect one. In a recent issue we illustrated a mechanically worked interlocking system of switches in use at the Grand Central Depot, New York, which was constructed and installed by the Union Switch and Signal Co., of Pittsburgh, Pa. This system represents the most advanced type of mechanical appliance for railroad safety. In our present issue we illustrate a plant recently erected by the same company for the blocking of a track, in which the work is done by electricity and pneumatic pressure combined, and in which all the operations are automatic, human agency having no part whatever in the work. The system illustrated has recently been introduced upon the Central Railroad of New Jersey, between Jersey City and Bergen Point. It is a four-track road, and is traversed by a very large number of trains daily. It has been found advisable to protect all classes of traffic, and accordingly the system has been put in operation on the four tracks.

As a very large number of trains, running at high speed, pass over this line, the blocks have been made short, ranging from 1,000 to 2,500 feet in length. At the commencement of each block signal posts are erected, one for each of the tracks. Each post carries two signals and protects the tracks over which it stands.

The signals are semaphores, and are arranged one above the other for a single line of track. The upper semaphore has a square end; the lower semaphore is fish-tailed in shape. When the upper one projects at right angles from the post, it indicates that a train is on the next block. Whenever it projects in this way, the lower one is also set at danger or caution. As the train leaves the block thus protected the upper signal falls, but the fish-tail signal remains at caution until the next block is passed and the train is two blocks distant. The upper signal is called the "Home" signal, the lower one the "Distant" signal.

The semaphores, whose construction is seen in the cut upon this page, are connected with a counterpoise, so that when left to themselves the counterpoise drops and sets them in the "Danger" position, or at right angles to the sustaining post. Directly below each semaphore a pneumatic cylinder, with single-acting piston, Fig. 2, is mounted, which is connected, by means of a balance lever and connecting rod, to the semaphore arm. As arranged, the piston, which works within the cylinder, is pressed upward to its highest position when the semaphore is at "Danger;" this, therefore, being the natural position of the whole apparatus. At the top of the cylinder a valve is arranged which can be opened electrically, and which closes automatically by a spring. To this valve a pipe is connected which communicates with a supply of compressed air. Above the valve is an electro-magnet, whose armature is connected to the valve stem. From what has been said it will be easily understood that, if a current of electricity is sent through the magnet, the valve will be opened. Compressed air will be admitted above the piston, which will be depressed, and as it goes down will force the semaphore in opposition to the counterpoise weight into the "Safety " position. The object to be attained, therefore, is this: When the train is on the block in advance of a set of signals, it must automatically cut off the current of electricity from both, so that they will both be drawn into " Danger" position. When the train is on the next block, the current of electricity must be again permitted to pass through the upper semaphore magnet, forcing it into "Safety" position; but no current must be admitted to the lower semaphore magnet until the second block has been passed.

Assuming, therefore, a supply of compressed air with battery to be given, the connections are carried out on the following basis: The rails for one block are insulated from the rails of the preceding and following blocks. At one end the right and left hand rails are connected by a wire containing in its circuit two cells of gravity battery. At the other end the same rails are connected through an ordinary relay. The battery is placed upon the end of the block first touched by the train. As long as no train is on the block, a current will pass through the relay, whose armature will be attracted, making a constant contact for the current of a local battery. When a train enters upon the track, its wheels and axles short-circuit the battery, so that the relay is thrown out of action and the circuit of the local battery is broken. To simplify the matter we will assume, first, that only one set of signals, the " Home " semaphores, are to be worked. The arrangements for this are shown in the diagram, Fig. 5. A local battery of eight cells is arranged in circuit with the magnet of the semaphore and with the relay, so that when the latter is excited a current passes through the semaphore magnet, the local battery circuit being closed by the relay.

When no train is on the block, it will be seen that the relay is in action; it closes the local battery circuit. The current from the local battery going to the semaphore magnet forces open the valve, compressed air is admitted into the semaphore cylinder, and the piston is driven down and the semaphore is forced into the "Safety" position. When a train enters on the block, the relay being thrown out of action as described, the local battery ceases to act upon the semaphore magnet, and the semaphore is drawn by the weight into "Danger" position.

Where the "Home" and "Distant" signals have both to be operated under the conditions described as in use upon the railroad we are describing, a somewhat more complicated arrangement of circuits is required, which is shown in the diagram, Fig. 6. The line battery and relay are identical with what has been described. The local battery circuit includes, as shown in the diagram, within its circuit a "Distant" signal at the commencement of one block, and the "Home" signal at the commencement of the next block in advance. To save wire a ground circuit is used as the return. Thus, when a block is occupied by a train, it sets, by the action just described, the "Distant" signal one block behind it at " Caution," and the "Home" signal directly behind it at " Danger" by short-circuiting the single relay. The lower end of the piston rod of the "Home" semaphore is provided with a circuit-closing switch, indicated conventionally in Fig. 6, under each upper signal. One terminal of this switch is connected to ground, the other terminal is connected to the wire leading to the magnet of the "Distant" signal below it. The circuit closer closes the circuit when the piston is in the upward position or when the "Home" semaphore is at "Danger." This establishes a ground connection for the line leading to the "Distant" semaphore magnet below it, which connection is of zero resistance. This operates as a shunt to the "Distant" semaphore magnet, so as to throw it out of action. It will be remembered that when a magnet is out of action its semaphore, is set at "Danger." Therefore when a "Home" signal is set at "Danger" by the passage of a train, the "Distant" signal below it is set at "Caution" by this action of the circuit-closing switch, and the "Distant" signal one block in its rear is also set at " Caution," because it is in circuit with the "Home" signal magnet in question.

It is obvious that where electricity and compressed air are depended on as the agents for the working of an automatic system, failures in their action are to be anticipated. In the present system, should electricity or pneumatic action fail, the signals affected would at once rise to the "Danger" position. It is here that its peculiar safety appears. The signals are only maintained at safety by the perfect working of the apparatus. The instant anything happens, the semaphores disclose it to the runner of the first train passing, by the signals assuming the "Danger" position.

As regards the details of its installation, the batteries are established in underground structures, a sectional view of one of which we show in the cut, Fig. 3. The same cut shows the relay pole, upon which the relay boxes are placed. In order to supply compressed air, a plant of compressors is established which communicates by pipes with a series of reservoirs placed along the line at the foot of tire signal poles, and pipes are carried from these to tire semaphore signals. The ordinary sleepers are found to give sufficient insulation to the rails; the fish plates, however, do not form adequate connection between tire rails, so that iron wire is fastened by iron pins driven into the foot of the rails across each joint of a block. Between the blocks the rail ends are insulated as shown in Fig. 4. For ground or return circuit, the pneumatic air pipes are used. For night work powerful white lanterns are arranged on the uprights, which, in the danger position of the signals, are masked by colored glass carried by the rear end of the semaphore. For the "Home" signal the glass is red, for the "Distant" signal it is green. The same colors are used to paint the sides of the semaphore board nearest the advancing engine. The other sides are painted white.

Enough has been shown to indicate the possibilities of the pneumatic system. It is applied also to moving switches, and a full interlocking switch and signal system utilizing electricity, hydraulic and pneumatic pressure, is now in use in track yards, which plant is installed by the same company. This system we shall probably illustrate in our next issue.