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. Increasing width of track centres of or more makes it much easier to mount signals and overhead wiring structures. Very wide centres at major bridges can have military value. It also makes it harder for rogue ships and barges knocking out both bridges in the same accident. Railway lines in desert areas affected by sand dunes are sometimes built on alternate routes so that if one is covered by sand, the other(s) are still serviceable. If the standard track centre is changed, it can take a very long time for most or all tracks to be brought into line. Accidents On British lines, the space between the two running rails of a single railway track is called the "four foot" (owing to it being 'four foot something' in width), while the space between the different tracks is called the "six foot". It is not safe to stand in the gap between the tracks when trains pass by on both lines, as happened in the Bere Ferrers accident of 1917. Narrow track centres on the Liverpool and Manchester Railway contributed to a fatal accident on opening day. A US naval scientist and submarine pioneer, Captain Jacques, was killed getting out of the wrong side of a train at Hadley Wood in 1916. Narrow track centres contribute to "Second Train Coming" accidents at level crossings since it is harder to see the second train – for example, the accident at Elsenham level crossing in 2005. Temporary single track When one track of a double-track railway is out of service for maintenance or a train breaks down, all trains may be concentrated on the one usable track. There may be bi-directional signalling and suitable crossovers to enable trains to move on to the other track expeditiously (such as the Channel Tunnel), or there may be some kind of manual safeworking to control trains on what is now a section of single track. See single-line working. Accidents can occur if the temporary safeworking system is not implemented properly, as in: Brühl train disaster, Germany – 2000. Zoufftgen rail crash, France – 2006 Out-of-gauge trains From time to