We have dealt with the signalling arrangements pertaining to double
lines of railway, showing that a system of signals is necessary to
prevent a train running into the back of its predecessor. Where trains
in both directions pass over a single line, not only has this element of
danger to be dealt with, but also the possibility of a train being
allowed to enter a section of line from each end _at the same time_.
This is effected in several ways, the essence of each being that the
engine-driver shall have in his possession _visible_ evidence of the
permission accorded him by the signalman to enter a section of single
line.

A SINGLE TRAIN STAFF.

The simplest form of working is to allocate to the length of line a
"train staff"--a piece of wood about 14 inches long, bearing the names
of the stations at either end. This is adopted where only one engine is
used for working a section, such as a short branch line. In a case like
this there is obviously no danger of two trains meeting, and the train
staff is merely the authority to the driver to start a journey. No
telegraphic communication is necessary with such a system, and signals
are placed only at the ends of the line.

TRAIN STAFF AND TICKET.

On long lengths of single line where more than one train has to be
considered, the line is divided into blocks in the way already described
for double lines, and a staff is assigned to each, the staffs for the
various blocks differing from each other in shape and colour. The usual
signals are provided at each station, and block telegraph instruments
are employed, the only difference being that one disc, of the key
pattern, is used for trains in both directions. On such a line it is, of
course, possible that two or more trains may require to follow each
other without any travelling intermediately in the opposite direction.
This would be impossible if the staff passed uniformly to and fro in the
block section; but it is arranged by the introduction of a train staff
_ticket_ used in conjunction with the staff.

No train is permitted to leave a staff station unless the staff for the
section of line to be traversed is at the station; and the driver has
the strictest possible instructions that he must _see_ the staff. If a
second train is required to follow, the staff is _shown_ to the driver,
and a train staff ticket handed him as his authority to proceed. If,
however, the next train over the section will enter from the opposi