In the instrument shown in Fig. 511, the upper arm of the miniature signal-post is coloured RED, and is moved by electricity through the medium of the block telegraph instrument in the signal-cabin in advance; and until this RED signal be lowered to the _line clear_ position by the signalman in the cabin in advance, no train must be allowed to start from or pass the cabin in the rear. The lower signal-arm coloured WHITE is lowered by the plunger A on its own instrument by the signalman in charge, and at the same moment lowers by electricity the upper or RED arm of the block-telegraph instrument in the signal-cabin at the other end of the section. The lower or WHITE arm is thus restricted to the signals sent away from the signal-cabin, while the upper or RED arm is restricted to signals received in the signal-cabin. In the centre there is a round handle B, which rotates a circular disc inside the instrument, and on this disc are painted three distinct train inscriptions, only one of which can be seen at a time through the glazed opening. One inscription has the words ALL CLEAR painted in black letters on a WHITE ground; another has the words TRAIN ON LINE painted in white letters on a RED ground; and the third has the words TRAIN OFF, BUT SECTION BLOCKED painted in black letters on a GREEN ground. The instrument is considered to be in its _normal_ position when the GREEN inscription is in view, and both the miniature signal-arms raised to _danger_. Fig. 512 represents a portion of double line divided out into sections, or working blocks, between the stations B, C, and D. Each station is provided with the necessary block-telegraph instruments, and the usual distant, home, and starting semaphore signals. [Illustration: Fig. 511, 512, 513]