Inclines and curves in a track limit the speed of trains, and upgrades require high power. A railroad generally follows topographical contours, but in many places the contours are smoothed by excavations, or cuts, and embankments, or fills. Original construction costs are weighed against anticipated operating costs and revenues. Because U.S. railroads were built largely before the economy of the country was fully developed, the original costs were usually kept low. Extensive improvements were necessary in the 20th century to strengthen roadbeds and eliminate or reduce sharp curves and heavy grades to permit higher speeds, heavier loads, and more frequent operation of trains.
Today, a 1 percent grade, or an incline rising 1 m in 100 m of horizontal distance, is considered steep; gradients on heavy-duty lines are usually limited to 0.5 percent. With the coming of fast freight service and of streamlined passenger trains capable of speeds of 160 km/m (100 mph) or more, curves received even more attention than grades.
Curvature is described in terms of the angle formed by radii meeting the ends of an arc that subtends a chord 31 m (100 ft) long. The maximum curvature on a given section of line varies, but it is generally set at 1.5 degrees, and in some cases 0.5 degree is the maximum curvature.
To avoid the jolting of trains, simple curves, which are arcs of circles, are approached by easement curves, in which the radius gradually decreases in length. To counteract centrifugal force, which causes a train to lean outward on a curve, the rails are banked; that is, the outside rail is laid higher than the inside rail, the degree of relative elevation depending on the sharpness of the curve and the expected speed of trains
The roadbed, which is also called the subgrade, must be carefully prepared before track is laid. To ensure stability, fills are built up in layers, each layer of earth, gravel, or other material being packed down thoroughly before the next is added.
The sides of both cuts and fills must slope gently enough to prevent slides, the angle depending on the type of material; the sides may be relatively steep if a cut is made through stone. To minimize erosion that might lead to cave-ins, earth sides are often covered with sod or with a thick layer of cinders.
The greatest damage suffered by roadbeds is caused by water. In cuts and sometimes on fills the shoulders of the roadbed are bordered by drainage ditches. Additional ditches intercepting and draining those that parallel the track, or systems of subsurface drainage pipes under the track, are sometimes needed. In some cases the track is laid on concrete slabs supported by timber piles. One way to keep ground moisture from softening roadbeds is to lay a heavy sheet of plastic material between the roadbed and the soil.
Where a railroad crosses depressions deeper than 15 to 18 m (50 to 60 ft), trestles, bridges, or viaducts are commonly used instead of fills. Track-improvement programs generally include the widening of roadbed shoulders and the strengthening of trestles and bridges. Tunnels are extremely expensive and are therefore avoided when track can be routed around a hill or mountain. Unless a tunnel is cut through solid rock, it must be lined with timber, brick, reinforced concrete, or corrosion-resistant metal. Sometimes a tunnel is built on a slight grade to ensure drainage.