Under the bridge supervisor are organized "bridge gangs," each consisting of a competent foreman with carpenters and laborers skilled in bridge work and living in "house" or "boarding" cars, and provided with pile-drivers, derricks, and all appliances for handling heavy timbers and erecting, tearing down, and repairing bridges. These cars form a movable camp, going from place to place as needed, and being side-tracked as near as possible to the work of the gang. Long experience begets great skill in their special duties, and the feats which these gangs will perform are often more wonderful than many of the more showy performances of railroad engineering. It is an every-day thing with such gangs to take down an old wooden structure, and erect in its place an iron one, perhaps with the track raised several feet above the level of the original, while fifty trains pass every day, not one of which will be delayed for a moment. [Illustration: A Rotary Steam Snow-shovel in Operation. (From an instantaneous photograph.)] Each of the supervisors of road has his assigned territory divided into "sections," from five to eight miles in length. At a suitable place on each section are erected houses for a resident section-master and from six to twelve hands. These are provided with hand- and push-cars, and spend their whole time in keeping their sections in good condition. Upon many roads annual inspections are made and prizes offered for the best sections. At least twice a day track-walkers from the section-gangs pass over the entire line of road. To simplify reports and instructions, frequently every bridge or opening in the track is numbered, and the number displayed upon it; and every curve is also posted with its degree of curvature and the proper elevation to be given to the outer rail. The work of the section-men is all done under regular system. In the spring construction-trains deliver and distribute ties and rails on each section, upon requisitions from supervisors. Then the section-force goes over its line from end to end, putting in first the new ties and then the new rails needed. Next the track is gone over with minute care and re-lined, re-surfaced, and re-ballasted, to repair the damages of frost and wet, the great enemies of a road-bed. Then ditches, grass, and the right-of-way have attention. These processes are continually repeated, and especially in the fall in preparation for winter. During the winter as little disturbance of track is made as possible, but ditches are kept clean, and low joints are raised by "shims" on top of joint ties. Essential parts of the equipment of any large road are snow-ploughs (pp. 154-5-6) and wrecking cars, with powerful derricks and other appliances for clearing obstructions. When wrecks or blockades occur these cars, with extra engines, section-hands, bridge gangs, and construction-trains, are rushed to the spot, and everything yields to the work of getting the road clear.