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this is just a nerdy question, but now I have started catching trains in the winter dark I have noticed that the refurbed HSTs only have one of their "main beam" headlights lit at any one time.  This is a feature that seems to occur on new trains also such as the Voyager.

I had assumed that the bulb had gone onthe unlit side, but it happens so often that it must be deliberate.

The setup seems to be LED "sidelights" lit on both sides of train but Halogen (?) very bright "main beam" light lit on one side only. 

My guess is that as the new lights are so bright only one of them is needed to illuminate the track ahead for the benefit of the driver.   The LED lights must be deemed to provide suffient light to illuminate the train for the beneift of track workers /trespasers/ crossing users (I assume that a single light would not be sufficient for this as it "looks less like a train" than a pair of lights - a single light is more likely to be mistaken for something else).  Does anyone know if this is right?  It it always the same light that is lit or can illumination be swapped to the other side (this would be an example of sensible redundancy if it were the case because a train will be failed if it is not properly lit)?

 

I believe it's because one is dipped (for night use) and the other full beam (daytime use).

http://groups.google.co.uk/group/uk.railway/browse_thread/thread/8c8380cf2c10e0cb/ (http://groups.google.co.uk/group/uk.railway/browse_thread/thread/8c8380cf2c10e0cb/)

Also note that headlights aren't intended for the train driver to see, but for the train to be seen.

It is even siller than you think the headlight is switchable by the driver for night and day time running. I can't remember which way round but it's something to do with not blinding track workers. Or at least that's what I've been told. Thus at this time of year we used to get trains at Taplow at dawn and dusk some with offside bright and others nearside bright as some trains started in the dark and other in the light.

Why don't we just adopt the the UICC triangle of equal intensity of lights? If you see three white lights in triangle coming towards you at 125 mph you are not going to think it's a car. Even where there is a toplight such as Voyagers and sheds you still have one bright headlight.

Now don't get me started on standardising couplings. Did any read the excellent article in October's Modern Railway outlining the mess we are in with interoperability of couplings?

Thanks everone for the info.  I see my assumptions were way too simple.

having a light on one side only is a bit odd.  I bet that a significant number of passengers will think it is a fault.

Nothing new in the HST's only having one headlight on, when first introduced in the mid 70's they did start by having both headlights on but as eightf48544 said there were complaints from us poor chaps at track level of being dazzled (easily done I know  ;D )

The headlights are indeed not really intended for the driver to see where they are going but with the increase in the use of light reflective sign and hight vis clothing for track workers the headlights are essential part of the safe system of operating

One thing you may never see, I hope, is the headlights flashing (similar to the way car indicators flash together) as this is a warning like the continuous sounding of the horn that there is a hazard.

HST's are one of the only type of traction that did have both headlights on until recently that I know of? Turbos have only ever had one headlight on since they were built, and I don't think there is any difference between the night and day bulbs on them. The refurbishment of HST's with brighter lights means having both on would probably be too bright at night for track workers and punters at the station - and the 2nd headlight acts as a 'spare' should one of them fail.

This does seem to be a case where "joined up thinking" has gone by the board. If the lights are too bright they dazzle track workers  and drivers coming the other way. Remember the fuss when the Voyagers first came out over the brightness of their headlights? However, if they are too dim track workers can't see the train coming and the driver can't pick up the reflective signs, usually speed limits sign so very important, and track workers hi vis vests aren't glowing.

I still believe a triangle of equal intensity lights is the way to go. It can't be mistaken for a car or motorbike on an adjcent road or any other pair of white lights near the railway. It would be unique to trains. Maybe there should also be another look at reflective signs, speed limit signs used to be yellow stencilled number so they showed up as silhouettes. It didn't matter if they were graffitied  I've seen many reflective signs made almost illegible, particularly going into to Padd.