signals can be lowered so as to allow a through or non-stopping train to pass a block-telegraph station at full speed. Where the traffic is moderate, it may be sufficient to have 342 block-telegraph instruments at each of the stations, but with a very frequent train service it will be found necessary to divide the line into shorter sections, and erect signal-cabins and block-telegraph instruments at intermediate points between stations. The code of bell or gong signals is extended to include various matters in connection with the train-working. For example, when a DOWN train is passing cabin B at full speed, the signalman may observe that there is something wrong--a carriage or waggon on fire, a tail-lamp missing, or other irregularity. It is too late to stop the train with his own signals, but by means of his bell or gong he can call upon the signalman in cabin C to stop and examine the train, and the DOWN distant and home signals at C can be raised to _danger_ before the train reaches the cabin at C. In every block-telegraph signal-cabin there is a train-book in which the signalman has to write down the time and description of every arriving or passing train, and, as this book lies before him, he has a complete record of the train-working, with the particulars of the exact times when the _line clear_ signals were given, and also when the train arrived or passed his signal-cabin. To guard against the possibility of a signalman inadvertently giving _line clear_, or allowing another train to pass his cabin before the previous train had reached the signal-cabin in advance, some railways have adopted the lock and block system. By this arrangement the starting signal at any cabin is electrically and mechanically locked from the cabin in advance, and can only be released or lowered by the action of the outgoing train itself when passing over a treadle or other appliance connected with the rails of the running-line at the signal-cabin in advance. This method practically gives the train the complete control of the section; and any signalman attempting, in error, to lower his starting signal would find it to remain fixed to _danger_ and immovable, until released by the arrival of the train at the advance cabin.