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'''Absolute Block''' is a method of signalling for controlling the passage of trains over a [[Double track|double track railway]]. The presence of two lines allows two trains to proceed in different directions at the same time. Trains usually travel on the left hand side, similarly to how cars always drive on the left in the UK. Each track is identified by its normal direction of traffic, with the two directions being named "Up" and "Down", with "Up" generally being defined as the line leading towards the principal terminus, normally London. The Absolute Block system ensures that only one train may be in a section on each track at any given time.