This schedule is all that is needed so long as all trains are able to keep on time, and there are no extras. Trouble begins when regular trains cannot keep on schedule, or when extra trains have to be sent out, not provided for on the schedule. A diagram, or graphic representation of this schedule, upon a board or large sheet of paper, is an important feature of the office regulating train-movements. Twenty-four vertical lines divide the board into equal spaces representing the twenty-four hours of the day, numbered from midnight to midnight. Horizontal lines at proportionate distances from the top represent the stations in their order between the termini, represented by the top and bottom lines of the diagram. The course of every train can now be plotted on this diagram in an oblique line joining the points on each station line corresponding to the time the train arrives at and leaves that station. The cut on the opposite page will illustrate. It represents a road 130 miles long from A to N, with intermediate stations B, C, D, etc., at different distances from each other, and six trains are shown as follows: A passenger train, No. 1, leaving A at 12 midnight and arriving at N at 4.05 A.M. A fast express, No. 2, leaving N at 12.45 and arriving at A at 3.30. A local passenger train, No. 4, which leaves N at 1.15, runs to E by 4 A.M., stops there until 4.10, and returns to N by 7 A.M.; being called No. 3 on the return, as the direction is always indicated by the train-number's being odd or even. No. 5 is a way freight, leaving A at 12.05 and making long stops at each station. No. 6 is an opposing train of the same character. [Illustration: Diagram Used in Making Railway Time-Tables.] [Illustration: A lamp swung across the track is the signal to stop.] The diagram shows at a glance how, when, and where all these trains meet and pass each other, and where every train is at any moment. Should it be desired to send an extra train at any time, a line drawn or a string stretched on the board will indicate what opposing trains must be guarded against. For instance, to send an extra through in three hours, leaving A between 1 and 2 A.M., a trial line will show that Nos. 5, 2, 4, and 6 must all be met or passed, and as (on a single-track road) this can only be done at stations, the extra must leave at 1.35 A.M., pass No. 5 at E, meet No. 2 at F, No. 4 at I, and No. 6 at J. A dotted line on the diagram indicates its run, and that No. 2 is held at F for 5 minutes to let it pass. If the road is double-tracked, only trains going in the same direction need be regarded.[16] [Illustration: A lamp raised and lowered vertically is the signal to move ahead.]