Page:Graphic methods for presenting facts (1914).djvu/88

 distance curves like those shown in Fig. 65 are then plotted, one for the rear end and another for the front end of each train. As the train remains constant in length throughout one whole journey it is obvious that the two curve lines must be a constant vertical distance apart throughout the length of the chart. At stations, subway trains must stop in such a way that the whole length of the train will be opposite the platform so that passengers may get in and out of every car. The trains follow each other very rapidly, and it is essential that sufficient room be left between two trains for safety. If two curves on a time-distance chart should touch each other, it would indicate a collision. The distance between different train curves on a chart gives information regarding the kind of signal and brake systems that must be used to give the desired degree of safety in operating the road.