I'm sure the answer is simple, but why do trains have one headlight on one side and just a marker light on the other? How come it isn't a standard 2 like cars?
The answer like many of this kind is to be found back in the 1970s.
With the continuing introduction of MAS the BRB decided that there was no further need for trains to display a reporting number, although this was quite controversial with us on the ground who firmly believed the opposite, for a number of reasons.
At night the reporting box indicators had provided a poor but visible source of frontal lighting to an otherwise black train, and this did at least give you some warning. On a good night, in the right conditions, you could see a reporting number from up to a mile away. Street lighting and other light pollution however meant that this was reduced significantly in urban areas.
This resulted in the decision subsequently to modify the boxes to contain light units, normally consisting of two lamps for redundancy. The reason for this was to make the train more visible at night to staff on the line.
With the introduction of the Class 87s from 1973 the standard was for two marker lights which were to be lit during the day and a centrally positioned high intensity lamp which was to be used at night.
Initially the light was located around buffer level as this was the closest to eye level however experience quickly revealed that visibility was improved with the lamp placed at a higher level and thus with the Class 56s in 1976 the lamp was placed towards the bottom of the cab windows.
By now the Standard required two marker lights together with a high intensity bulb for night time use on all new build stock. Existing lococs with a reduced life expectancy were permitted to operate with two frontal lights only, other locomotives were modified during main works to be equipped with the new headlamp arrangements.
From the early 1990s the high intensity lamps were altered so that they focussed towards the left hand side with the beam providing illumination up to about 100 yds ahead of the traction unit.
The reason for this was two-fold, firstly to assist the driver when examining the line, and secondly to assist in illuminating the new standard retro-reflective lineside warning signs.
The current frontal light standard reflects the UIC requirements.