in the way must wait
till the flyer has passed. When anything of this sort occurs the whole
plan has to be changed, and all trains have to be run on a new schedule
that must be made up on the moment.

The ideal train schedules, or those by which the systems are regularly
governed, are charted out beforehand on a ruled sheet, as a ship's
course is charted on a voyage, in the main office of the railroad. Each
engineer and conductor is provided with a printed copy in the form of a
table giving the time of departure and arrival at the different points.
When the trains run on time it is all very simple, and the work of the
despatcher, the man who keeps track of the trains, is easy. When,
however, the system is disarranged by the failure of a train to keep to
its schedule, the despatcher's work becomes most difficult. From long
training the despatchers become perfectly familiar with every detail of
the sections of road under their control, the position of every switch,
each station, all curves, bridges, grades, and crossings. When a train
is delayed and the system spoiled, it is the despatcher's duty to make
up another one on the spot, and arrange by telegrams, which are repeated
for fear of mistakes, for the holding of this train and the movement of
others until the tangle is straightened out. This problem is
particularly difficult when a road has but one track and trains moving
in both directions have to run on the same pair of rails. It is on roads
of this sort that most of the accidents occur. Almost if not quite all
depends on the clear-headedness and quick-witted grasp of the
despatchers and strict obedience to orders by the trainmen. To remove as
much chance of error as possible, safety signalling methods have been
devised to warn the engineer of danger ahead. Many modern railroads are
divided into short sections or "blocks," each of which is presided over
by a signal-tower. At the beginning of each block stand poles with
projecting arms that are connected with the signal-tower by wires
running over pulleys. There are generally two to each track in each
block, and when both are slanting downward the engineer of the
approaching locomotive knows that the block he is about to enter is
clear and also that the rails of the section before that is clear as
well. The lower arm, or "semaphore," stands for the second block, and if
it is horizontal the engineer knows that he must proceed cautiously
because the second section alre