Fog, of course, is not the only cause of bad visibility, so when reading a railway Rule Book you will frequently come across the phrase "in fog or falling snow" as the latter is just as difficult to work in as the former.

This article is not meant to be the full ins and outs of operating in conditions of impaired visibilty, more a general overview of what is involved from the point of view of a Signalman or Traincrew.

Many of the rules developed down the years are to do with fog situations in areas of semaphore signalling. One inherent advantage of the colour light signal is that its high-intensity lamp makes it far easier to see.

From the Signalman's point of view one of the first results of fog is a reduction of the area where he can safely shunt, or otherwise deal with trains. Each box will have its "fog clearing points", far more restrictive than his normal clearing points, which will affect his ability to accept a train from the next box up the line. In addition, where a Signalbox is between two others with trains running straight through all sections, whilst fog working is in place the Signalman has to have received the "Train Out of Section" for the previous movement from the box ahead before he can accept a train from the box in his rear.

Once it has been decided that visibilty is bad enough to introduce fog signalling, one of the first considerations is whether or not a Fogsignalman (or men) is required. All Drivers know where every signal is, and they have the responsibility to check each one that they approach, and act according to how they find it. However, if a signal is on a tall post and the fog or snow very thick, it can be almost impossible to see the arm which would mean the driver would have to stop his train and inspect the signal at close quarters, possibly ringing the Signalman as well. To ease this situation a Fogsignalman can be appointed to stand by the signal, equipped with a supply of detonators, a lamp and flags. When the signal shows a Danger (or Caution) aspect he has to place one or two detonators (depending on the Rules governing that stetch of line) on the same rail and show a Danger or Caution hand signal. When the signal is pulled off he must then remove the detonators and exhibit a proceed (green) signal. If he has to leave his post for any reason, e.g. to obtain a fresh supply of detonators, then he must leave just one detonator on the line. If he is posted to a signal with both a Danger and a Caution signal on the same post, then he must keep his detonators on the line whilst the stop signal is at Danger, must exhibit a yellow Caution signal when just the stop signal is off and must exhibit the green proceed signal only whilst both signals are off. Any Driver running over a detonator must stop his train immediately and investigate the reason why.

As was said above, all Drivers know where every signal is, and they have the responsibility to check each one as they approach it, but in times of thick fog it is very easy to become disorientated as, in the absence of a friendly bridge or similar, well-known features that the Driver will look out for simply cannot be seen. When carried out correctly, fog signalling should ensure the safety of all concerned but when things go wrong in fog then the result can be horrific, such as the carnage at Lewisham on 4th December 1957 when the Driver of 34066 Spitfire, hauling a Cannon Street to Ramsgate train, failed to see or stop at a red signal and ran into the back of a suburban electric service, compounded by a bridge then collapsing. 90 people lost their lives and another 109 were injured.