Semaphore signal question

[Raul_Duke]

Apologies if this is in the wrong place, but does anyone know why the rear of a semaphore arm is painted white with a stripe / chevron rather than just plain black?

Surely this just makes it more confusing.

 

[LowLevel]

I don't know if it's the actual answer having never thought about it but the signalman has to check the arm is on or off if repeaters aren't provided in the box - if the box is in advance of the signal having the arm painted black would make this rather tricky!

 

[godfreycomplex]

I don't know if it's the actual answer having never thought about it but the signalman has to check the arm is on or off if repeaters aren't provided in the box - if the box is in advance of the signal having the arm painted black would make this rather tricky!

Correct. At night this is done by means of a "back speck" (a small light that's only visible when the signal's off); but for daytime visibility it is white.

 

[D Foster]

The white with a vertical black stripe on a semaphore Stop signal's arm is indeed so that the Signalman/Signaller can see the arm clearly - when it is a signal on the approach side of the Box (aka "In Rear of" the Box). It confirms both that it is in fact still there and its position.
At night the back light only shows when the signal is ON - at Stop/Danger. Not when the signal is off. This provides that the Signalman can see that signals with their backs to him (those In Rear of the Box) are showing a stop aspect and that the light is burning. This provides a fail-safe - should the light go out or the signal not be showing a red aspect at night for any other reason - then no light shows through the back light. When the signal is off the back light is obscure by a back-light blinder - a metal plate attached via a casting to the rear end of the signal arm pivot rod so that it moves in exactly the same way as the arm and exactly duplicates its position. Therefore when ever the arm is not correctly On the blinder obscures the back light.
Where the view of the back of a signal is obstructed or too far away to be seen (as with a Distant signal) a repeater is required in the Box. As far as practical signal repeaters are placed on the Block Shelf directly above the lever for the signal that they repeat.

Back lights are always white and are not aspects.

Both the daytime arm and the night time backlight can be used by other staff to note when a movement is signalled. You cannot do this with a Colour Light signal! At least - not during the day... However - the incandescent lamp Colour Lights could reflect off of structures - such as OLE stanchions) at night. This doesn't happen with LED signals.

 

[DerekC]

This doesn't happen with LED signals.

As a matter of interest - why not?

 

[edwin_m]

On occasion a train would have to stop beyond the starting signal, or a loco being attached at a terminus might not fit between the train and the signal, due to short platforms. The driver could then check the back of the signal to ensure that it was (still) clear before starting away.

Incidentally some of the early colour light schemes had double-sided theatre indicators on the signals protecting station throats, displaying in orange for the "wrong way". I assume this was so track workers could see what route was being set across the junction.

 

[Surreytraveller]

Correct. At night this is done by means of a "back speck" (a small light that's only visible when the signal's off); but for daytime visibility it is white.

I thought it was when the signal's on? Otherwise its not fail safe?

Edit - just seen D Foster's post!

 

[Surreytraveller]

As a matter of interest - why not?

Perhaps the LEDs are more focused than incandescent lamps?

 

[alxndr]

However - the incandescent lamp Colour Lights could reflect off of structures - such as OLE stanchions) at night. This doesn't happen with LED signals.

I've definitely seen LEDs reflect off OLE. Perhaps less so than lamps might have done (I've never seen them shine onto OLE), but the metalwork took up a definite red/green glow.