Because of the elaborate system of trenches, and the distance from the front line to that part of the country where it was safe to operate in the open, it was necessary to build railways which would travel through tunnels and communicating trenches to the front lines. These were narrow-gage railroads and a special standard form of track section was designed, which was entirely of metal, something like the track sections of toy railroads. The tracks were very quickly laid and taken up at need. The locomotives had to be silent and smokeless and so a special form of gasolene locomotive was invented to haul the little cars along these miniature railroads to the front lines. Usually the trench railroads did not come to the very front of the battle-line, but their principal use was to carry shell to the guns which were located in concealed positions. Railroad or tramway trenches could not be sharply zig-zagged but had to have easy curves, which were apt to be recognized by enemy airplanes, and so they were often concealed under a covering of wire strewn with leaves. PERISCOPES AND "SNIPERSCOPES" But while the armies were buried underground, it was necessary for them to keep their eyes upon each other so that each might be ready for any sudden onslaught of the other. Snipers were always ready to fire at any head that showed itself above the parapet of the trench and so the soldiers had to steal an idea from the submarines and build them periscopes with which they could look over the top of their trenches without exposing themselves. A trench periscope was a very simple affair, consisting of a tube with two mirrors, one at the top and one at the bottom, set at such an angle that a person looking into the side of the tube at the bottom could see out of the opposite side of the tube at the top. Observation posts were established wherever there was a slight rise in the ground. Sometimes these posts were placed far in advance of the trenches and sometimes even behind the trenches where it was possible to obtain a good view of the opposing lines. Sometimes a tunnel would be dug forward, leading to an outlet close to the enemy's lines, and here an observer would take his position at night to spy with his ears upon the activities of the enemy. Observers who watched the enemy by day would often not dare to use periscopes, which might be seen by the enemy and draw a concentrated fire of rifles and even shell. So that every manner of concealment was employed to make the observation posts invisible and to have them blend with their surroundings. Observers even wore veils so that the white of their skin would not betray them. [Illustration: Redrawn from Military Map Reading by permission of E. C. McKay FIG. 1. A "sniperscope" with which a sharpshooter could take aim without showing his head above the parapet]