signalisation d'espacement

OBS: La signalisation moderne d'espacement des trains est le block automatique lumineux, ou BAL, à trois indications, (sémaphore, avertissement, voie libre) ou, sur les lignes à grand débit parcourues à vitesse élevée, à quatre indications (sémaphore, avertissement, feu jaune clignotant, voie libre).

CONT: The multi-aspect signalling commonly used in the UK today is a 4-aspect system. It works similarly to the 3-aspect system except that two warnings are provided before a red signal, a double yellow and a single yellow. This has two purposes. First, it provides early warnings of a red signal for higher speed trains or it can allow better track occupancy by shortening the length of the blocks. The high speed trains have advanced warning of red signals while the slower speed trains can run closer together at 50Km/h or so under ""double yellows"".