ROADBEDS AND STRUCTURES Of the two major classifications of railway property, stationary or fixed property (including roadway and structures) and rolling stock (cars and locomotives), the former comprises somewhat more than 75% of the total investment. Comprehended under the subject of roadway and its construction are the important topics of clearance, roadbed design and construction, culverts, and the ballast bed. Track structure and special trackwork embracing ties, rails, fastenings, etc., are also here treated with relation to present and future design. Signs, fences, crossings, labour-saving equipment, railway timbers, water stations, are likewise discussed briefly as integral parts of the subject of stationary property. (See also BRIDGES.) An increasing concentration of traffic, greater individual axle loads, and the higher speeds of freight trains have imposed heavier burdens upon the roadway and track structure. The ton miles per mile of road were t00% more in 1928 than in 1903 on American railroads. Driving axle loads, which in 190o did not average more than 30,000 lb., are now in excess of mom lb. in some cases.
The development of a more stable roadbed structure has been dictated by necessity as well as to reduce track maintenance to increase the margin of safety for anticipated heavier axle loads, and to effect a decrease in train resistance. Practi cally all American railroads have strengthened the roadway by widening it, increased the depth of the ballast, the number of ties in a panel, the weights of rail, as well as making greater use of tie plates, anti-creepers and other auxiliary fastenings.
Clearance.—Since 1918 changes have been made inthe di mensions and types of equipment, in the sizes and weights of shipments, and in the clearances fixed by legislative acts in many of the States. The magnitude of the problems involved has led to the standardization of clearances for new (single) track con struction, for bridges and tunnels, and for the third rail of elec trical tracks; and a minimum distance between tracks has also been established. Consideration has likewise been given to struc tures placed along side tracks, a minimum distance being estab lished from the gauge line of the rail on tangents.
the improvement of the roadbed with its cushion of ballast has been in progress since the beginning of railways, its most marked developments have taken place since the beginning of the present century. The minimum width (1929) for both excavations and embankments is 20 ft., with the added provision that a roadbed shoulder of not less than 18 in. be maintained outside of the toe of the ballast slope. Crowning of the roadway is a common prac tice to facilitate drainage; while the making of fills of clay or other impervious materials is avoided. Ditches have been deep ened to provide for sub-draining the roadbed, as well as for dis charging the storm water. Intercepting ditches are coming more into use for diverting from cuts water which is collected over a considerable drainage area, and for conveying it along the foot of embankments, avoiding in the first case the deposit of material which may interfere with traffic, and, in the second case, erosion which may endanger the stability of the roadway. Sub-drainage of cuts, by means of vitrified pipes laid with open joints and fed by perforated transverse pipes, small tiles or cinder-filled trenches, is being more generally employed as a means of effecting large savings in track surfacing costs.