See also: mailbox

The automatic block luminous ( mailbox ) is an automatic system of indication used by the SNCF to ensure the spacing of the Train S circulating on the same way. It calls upon light signals placed at the entry of each canton, and at circuits track making it possible to change it the aspect according to the advance of the trains.

This system is established on the principal lines with strong traffic, with a limited length of canton (about 1500 m with a maximum of 2800 m), which authorizes an important flow.

The signals consist of panels (target) at black bottom and white edging carrying several fires. The principal indications met in mailbox are the following ones:

• a green light: free track. The mechanic is authorized to circulate moving normal, if nothing is opposed to it.
• a yellow fire: warning. The mechanic must be able to stop with the next signal of stop or stop.
• a red light: semaphore. The mechanic must stop before the signal.

The semaphore - signal of quartering or block - can possibly be crossed after the stop under certain applicable conditions according to:

• the type of plate associated with the signal (BM, PR, or F);
• rules specific to the line.

It is presented in the form of a red light (in luminous indication).

Its significance should not be confused with that of the square.

The square - signal of protection - is insuperable. It is presented in the form of two red lights, aligned vertically or horizontally (in luminous indication).

There is a possibility of confusion between semaphore and square, which already was at the origin of accidents: a signal posting a square, and of which one of two red lights is accidentally extinct.

Since the square should not be crossed , the panels which can post the square have a Nf plate (not-passable), and a white small fire - the eyepiece of crossing, which ignites when the signal posts a semaphore, and it to confirm to the mechanic that it is not a square with a roasted fire, but well a semaphore (only one red light).

On the image above, there is an eyepiece of crossing whereas it is a panel of semaphore. Formerly the eyepiece was also added to the panels F and always lit. It was superfluous because, by observing a red light, the mechanic is supposed in any case to examine the plate associated with the signal. Today, the panels F do not carry any more an eyepiece, except those on bracket, where the plate is less easily readable the night.

Moreover one sees on the image that the eyepiece is extinct; this one was not dismounted.

External bond

• Principles and description of railway Indication the standard SNCF
• railway Indication the SNCF