placement of station buildings and the directionality of travel demand. Handedness of traffic can affect locomotive design. For the driver, visibility is usually good from both sides of the driving cab so the choice on which side to site the driver is less important. For example, the French SNCF Class BB 7200 is designed for using the left-hand track and therefore uses LHD. When the design was modified for use in the Netherlands as NS Class 1600, the driving cab was not completely redesigned, keeping the driver on the left despite the fact that trains use the right-hand track in the Netherlands. Generally, the left/right principle in a country is followed mostly on double track. On steam trains, the steam boiler often obscured some view, so the driver preferably was placed nearest to the railway side, so signals were seen better. On single track, when trains meet, the train that shall not stop often uses the straight path in the turnout, which can be left or right. Bi-directional running Double-track railways, especially older ones, may use each track exclusively in one direction. This arrangement simplifies the signalling systems, especially where the signalling is mechanical (e.g. semaphore signals). Where the signals and points (UK term) or rail switches (US) are power-operated, it can be worthwhile to signal each line in both directions, so that the double line becomes a pair of single lines. This allows trains to use one track where the other track is out of service due to track maintenance work, or a train failure, or for a fast train to overtake a slow train. Crossing loops Most crossing loops are not regarded as double-track even though they consist of multiple tracks. If the crossing loop is long enough to hold several trains, and to allow opposing trains to cross without slowing down or stopping, then that may be regarded as double-track. A more modern British term for such a layout is an extended loop. Track centres 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