ines apply also to the working of single lines, with the addition that
not only are the distant, home, and starting signals interlocked with
each other, but with the signals and points governing the approach of a
train from the opposite direction--in other words, the signals 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 from the
opposite direction are at danger, 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 signalman ahead, the series of questions to be signalled being very
similar to those detailed for double lines. There is, however, one
notable exception. On long lengths of single line it is necessary to
make arrangements 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 more than one train shall not be on a section
of single line between two crossing-places it is laid down that, when a
signalman at a non-crossing station is asked to allow a train to
approach his station, he must not give permission until he has notified
the signalman ahead of him, thus securing that he is not asking
permission for trains to approach from both directions at the same time.
Both for single and double line working a number of rules designed to
deal with cases of emergency 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-frame in a signalling cabin
at Didcot.]

"POWER" SIGNALLING.

In a power system of signalling the signalman is provided with some
auxiliary means--electricity, compressed air, etc.--of moving the
signals or points under his control. It is still necessary to have a
locking-frame in the signal-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 th