If a westward train is represented in Fig. 11 at _T_ as approaching station _A_ to enter the block 2, both the distant signal _D_ and the home signal _S_ being at danger, the system is so arranged that the signalman at station _A_ cannot change those signals, i.e., to a position of safety, until the signalman at station _B_ permits him to do so. If the signalman at station _A_ desires to open block 2 for the entrance of the train _T_, he asks the signalman at station _B_ by wire to release the lock _l_ to enable him to do so. If there is no train in block 2, the signalman at station _B_ pushes the button _P′_ or “plunges” it. This raises the lock _l_ at station _A_ and the signalman immediately pulls the lever _L_ to its extreme leftward position, throwing both the signals _S_ and _D_ to the position of safety or clear, indicated by the dotted lines at _S_². At the same time the indicator _E_ at station _A_ shows the words “Clear to _B_,” while the slot _Fʹ_ at _B_ shows the words “On from _A_.” The signals at stations _B_ and _C_ are supposed to be in their normal position of danger, and the indicator _E′_ at station _B_ shows the words “Blocked to _C_.” The home and distant signals _Sʹ_ and _Dʹ_ are now at danger, but the train _T_ may enter block 2 and proceeds to do so, it being remembered that the signalman at station _A_ cannot move the lever _L_, as it has passed out of his control; not even the signalman at station _B_ can give him power to do so. The train _T_ now passes station _A_ into block 2. As the last car passes over the point _G_ its wheels strike what is called a track-treadle, an appliance having electrical connection with the lock _l_. The effect of the wheels of the last car of the train passing over the treadle at _G_ is to release lock _l_, enabling the signalman at station _A_ immediately to raise the arms _S_ and _D_ to the position of danger. It is to be observed that he cannot do this until the entire train has passed into block 2; nor, since his plunger is locked by the same treadle at _G_, can he signal “Safety” or “Clear” to the entrance of block 1. Hence no train can enter block 1 to collide with the rear end of the train just entering block 2. When the signalman at station _A_ has raised his signal _S_ to danger, it again passes out of his control, indeed out of both his control and that of the signalman at _B_, until the last car of the train passes over the treadle _Gʹ_ at the entrance of block 3. The train has now passed into block 2 and is approaching station _B_. The signalman at _B_ asks _C_ by wire to release the lever _Lʹ_, and if block 3 is clear, _C_ plunges at _P″_.