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 already has a train in it; if the upper arm
is straight the "runner" knows that a train or obstruction of some sort
makes it unsafe to enter the first block, and if he obeys the strict
rules he must stay where he is until the arm is lowered At night, red,
white, and green lights serve instead of the arms: white, safety; green,
caution; and red, danger. Accidents have sometimes occurred because the
engineers were colour-blind and red and green looked alike to them. Most
roads nowadays test all their engineers for this defect in vision.

In spite of all precautions, it sometimes happens that the block-signals
are not set properly, and to avoid danger of rear-end collisions,
conductors and brakemen are instructed (when, for any reason, their
train stops where it is not so scheduled) to go back with lanterns at