wear and tear on rolling stock caused by the small surface gap at the joints between rails can be counteracted; this also makes for a quieter ride. On curves, the outer rail may be at a higher level than the inner rail. This is called superelevation or cant. This reduces the forces tending to displace the track and makes for a more comfortable ride for standing livestock and standing or seated passengers. A given amount of superelevation is most effective over a limited range of speeds. Points and switches - also known as turnouts - are the means of directing a train onto a diverging section of track. Laid similar to normal track, a point typically consists of a frog (common crossing), check rails and two switch rails. The switch rails may be moved left or right, under the control of the signalling system, to determine which path the train will follow. Spikes in wooden ties can loosen over time, but split and rotten ties may be individually replaced with new wooden ties or concrete substitutes. Concrete ties can also develop cracks or splits, and can also be replaced individually. Should the rails settle due to soil subsidence, they can be lifted by specialized machinery and additional ballast tamped under the ties to level the rails. Periodically, ballast must be removed and replaced with clean ballast to ensure adequate drainage. Culverts and other passages for water must be kept clear lest water is impounded by the trackbed, causing landslips. Where trackbeds are placed along rivers, additional protection is usually placed to prevent streambank erosion during times of high water. Bridges require inspection and maintenance, since they are subject to large surges of stress in a short period of time when a heavy train crosses. Gauge incompatibility The use of different track gauges in different regions of the world, and sometimes within the same country, can impede the movement of passengers and freight. Often elaborate transfer mechanisms are installed where two lines of different gauge meet to facilitate movement across the break of gauge. Countries with multiple gauges in use, such as India and Australia, have invested heavily to unify their rail networks. China is developing a modernized Eurasian Land Bridge to move goods by rail to Western Europe. Train inspection systems The inspection of railway equipment is essential for the safe movement of trains. Many types of defect detectors are