The distance between the track centres makes a difference in cost and performance of a double-track line. The track centres can be as narrow and as cheap as possible, but maintenance must be done on the side. Signals for bi-directional working cannot be mounted between the tracks so must be mounted on the 'wrong' side of the line or on expensive signal bridges. For standard gauge tracks the distance may be or less. Track centres are usually wider on high speed lines, as pressure waves knock each other as high-speed trains pass. Track centres are also usually wider on sharp curves, and the length and width of trains is contingent on the minimum railway curve radius of the railway.