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A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers (''engineers'' in the US). The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop. == Application and positioning of signals == (詳細はtimetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it. Signals are used to indicate one or more of the following: * that the line ahead is clear (free of any obstruction) or blocked. * that the driver has permission to proceed. * that points (also called ''switch'' or ''turnout'' in the US) are set correctly. * which way points are set. * the speed the train may travel. * the state of the next signal. * that the train orders are to be picked up by the crew. Signals can be placed: * at the start of a section of track. * on the approach to a movable item of infrastructure, such as points/switches or a swingbridge. * in advance of other signals. * on the approach to a level crossing. * at a switch or turnout. * ahead of platforms or other places that trains are likely to be stopped. * at train order stations. 'Running lines' are usually continuously signalled. Each line of a double track railway is normally signalled in one direction only, with all signals facing the same direction on either line. Where 'bi-directional' signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「railway signal」の詳細全文を読む スポンサード リンク
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