The mode of operation is as follows: Suppose the sections C and C' to be entirely clear of cars; then the electric current from the positive pole P of the battery B will pass as indicated by the arrows _x x_, through the wire _k'_, rail _b_, wire _l'_, and magnet M, charging the same, and return through the wire _l_, rail _a_, and wire _k_, as indicated by the arrows _y' y_, to the negative pole N of the battery. The magnet M, being thus charged, attracts its armature and swings the signal-disk S into the position of concealment shown in dotted outline, and holds it in that position as long as the sections C C' are clear. Now let a train enter upon C or C', as indicated at H C', and the wheels and axles of the same will bridge over the rails _c_ and _d_, and thus, by offering a large conducting-surface, will present to the electric current a complete circuit, which offers much less resistance to its passage than that through the magnet M. The electricity now takes the course over the wire _k'_, rail _d_, wheels and axle H, returning, by the rail _c_ and wire _k_, to the battery, as indicated by the arrows _x x' y_, using the rails _c_ and _d_, as will be seen, with their bridge, and entirely avoiding the magnet M, which, being thus demagnetized, lets go its armature, and the counterpoise _w_, which slightly overbalances the disk S, carries the same up in front of the orifice, into a position of exposure, where it remains, as shown, while a train is on section C or C'. When, however, the train has run off, leaving sections C and C' clear, the magnet M is instantly charged again and the signal-disk is removed and kept concealed until the track is again blocked by the presence of another train, when the same process is repeated. When the signal-disk is in a position of exposure, as shown, the lever _l_ may serve to close an additional circuit through the battery B, which may be used to operate an alarm, I, in conjunction with the signal S, or to actuate another signal at a distant point. Furthermore, the concealment of the signal S may serve to close another circuit for exposing another signal, or the reverse. Instead of using the signal G, constructed as herein minutely described, a signal of any suitable construction may be used without affecting the spirit of the invention. Furthermore, instead of using the magnet M to actuate the signal directly, it may be used as a relay, operating, when charged, to keep the circuit which directly actuates the signal open or closed, as desired. It is evident that an alarm may be used either in conjunction with or independently of a visual signal. The drawing shows an application particularly adapted to road-crossing signals on a double track. The signals may be used, also, on a single track and be applied as block signals and for other purposes on single or double tracks. When used as a block-signal or for other purposes, it may be desirable to indicate at a distant station when the signal is operative.