nnection of the instruments.]

We may now watch the doings of signalmen in four successive boxes, A,
B, C, and D, during the passage of an express train. Signalman A calls
signalman B's attention by one beat on the tapper-bell. B answers by
repeating it to show that he is attending. A asks, "Is line clear for
passenger express?"--four beats on the bell. B, seeing that the line is
clear to his clearing point, sends back four beats, and pins down the
white key of his instrument. "Line clear" appears on the opening, and
also at that of A's keyless disc. A lowers starting signal. Train moves
off. A gives two beats on the tapper = "Train entering section." B pins
indicator at "Train on line," which also appears on A's instrument. A
places signals at danger. B asks C, "Is line clear?" C repeats the bell
code, and pins indicator at "Line clear," shown on B's keyless disc
also. B lowers all signals. Train passes. B signals to C, "Train
entering section." B signals to A, "Train out of section," and releases
indicator, which returns to normal position with half of each flag
showing at the window. B signals to C, "Train on line," and sets all his
signals to danger. C pins indicator to "Train on line." C asks, "Is line
clear?" But there is a train at station D, and signalman D therefore
gives no reply, which is equivalent to a negative. The driver, on
approaching C's distant, sees it at danger, and slows down, stopping at
the home. C lowers home, and allows train to proceed to his starting
signal. D, when the line is clear to his clearing point, signals "Line
clear," and pins indicator at "Line clear." C lowers starting signals,
and train proceeds. C signals to D, "Train entering section," and D pins
indicator at "Train on line." C signals to B, "Train out of section,"
sets indicator at normal, and puts signals at danger. And so the process
is repeated from station to station. Where, however, sections are short,
the signalman is advised one section ahead of the approach of a train by
an additional signal signifying, "Fast train approaching." The block
indicator reminds the signalman of the whereabouts of the train. Unless
his keyless indicator is at normal, he may not ask, "Is line clear?" And
until he signals back "Line clear" to the box behind, a train is not
allowed to enter his section. In this way a section of line with a full
complement of signals is always interposed between any two trains.

THE WORKING OF SINGLE LINES.