What is the purpose of a rail gap indicator..?

If the lights are lit, the current on the far side of the gap is OFF. I've heard the phrase 'cherries' used as in 'The cherries are on, mate'. Comes from the three red bulbs....

There are also rail gap indicator repeaters, to give advance warning.

... because you shouldn't try to go any further ;D

Does that mean then, that sometimes during traffic hours that the current is turned off? I assumed it would be on all the time until engineering hours. If it gets turned off during traffic hours what might be some reasons for this

Not quite a stop signal, because if unable to stop in time, Train Operators are authorised by the Rule Book to coast the whole of their train over the gap.

As to when you'd see one lit in Traffic Hours, examples are:

1) When current has been discharged due to spurious TT activation (like happened on the Northern Line between South Wimbledon and Morden today)

2) Traction current discharged due to incident / emergency (person on the track)

3) Defective RGI

4) Traction current discharged over a section to which points are set, but not the route the train will be taking (e.g. East Finchley Platform 2 early in the morning, when current is switched on for the northbound, but not yet for the southbound, the RGI will be extinguished only when the signal route is set and the points thrown).

5) Traction current not yet switched on to the section ahead because your train is running early (I've seen this happen at Finchley Central).

Those are probably the most common reasons, there are others.

To add one on to North End's list, RGI's also illuminate when they are too many trains in a section and the circuit breakers kick in.

A Rail Gap Indicator or RGI is a stop signal as it displays a red aspect to on coming trains, although it can't physically stop a train. Depending on the circumstances it can be a category A or B SPAD, and if you stall and bridge a rail gap you will 'juice up' or liven the dead section. They normally have plenty of sighting distance (including the addition of a repeater) so you should be able to stop prior to the RGI, but if traction current is discharged whilst you are approaching an RGI, as a T/Op you must decide if you can stop prior to the RGI or if you need to motor up to fully pass over the gap and not bridge the gap, which would result in the dead section becoming live. If you did bridge the gap you would be asking the controller to discharge the section at the rear, not to pass over it.

But surely that will (theoretically, at least) liven up the dead section whilst the train is passing over the gap?

I would also agree that it could be treated as a Category A or B SPAD, depending on the circumstances.

On the older stock you were advised to Motor up and then shut off prior to the gap, trip your 'MG's' and cut out the Compressor Governor in the leading cab. By doing this you are isolating all of the 630v equipment on the train from drawing current and reducing the chances of 630v being transfered to the 'dead' section via the train. On the modern stock you would put the TBC in 'off and release', trip your overloads and coast across the gap until you believe you have cleared the gap and signals permitting , try and reach the next station.

If a train passed an illuminated RGI that was attached to a signal post for a station starting signal that at the time was showing a green aspect (as RGI illuminating do not put signals back to danger) or was fixed to a post that was clearly visible to a T/Op who was berthed at a platform, then would criteria of a Category C SPAD still apply? You can never assume that passing a RGI illuminated at danger will always be classed as category C SPAD, as it depends on the location of the train at the time of the incident.

I'm pretty sure there are still some short gaps on the Northern Line. Lambeth substation gap springs to mind as one location. Also I'd suspect there are a few places where you pass from the n/b to s/b sections at crossovers.

Still plenty of locations around where there is no RGI, despite there being current rail gaps.

You obviously haven't driven a train a full speed then come around a corner and found one illuminated, leaving you very little decision and stopping time!! ;D ;D

As linecontroller66 has already said, not all traction current section gaps have RGI's. Where there are RGI's, they are only at the start of traction current section.

Section switches (to the casual observer this may look like two Yellow boxes side by side at intervals along the track) do indeed break traction current sections into smaller parts....however they are generally only used following some incident on the railway, or as part of the protection arrangements for engineering works - so RGI's are not warranted.

There are other types of switches that may also resemble a section switch, but the indication of operation is always the same - a red flag or lamp at the base, pointing across the track. Only if a train is required not to pass the switch will a second red flag or lamp be placed in the four foot (middle of the track).

Disclaimer - I must stress that I'm fairly certain I'm right in what I'm saying within this post regarding bus bars, earth faults and sectionalisation gaps, but I am open to correction.....

Traction current sections are usually double end fed - a fictitious example:

Sloane Square to Westminster. There will be a feed from both the Sloane Square and Westminster sub stations to both ends of the traction current section. The next traction current section could be Westminster to Blackfriars, with the same set up. There'll be a bus bar in Westminster sub station which means both traction current sections will get a feed off the same supply at Westminster sub station - and can pass earth faults from one traction current section to the next.

Sectionalisation gaps mean there is no bus bar in the sub station, and thus the two adjacent traction current sections are independently fed - they're completely separate and no faults can pass through.

Sectionalisation gaps can cover several traction current sections with both the line controller and the power control room having a tapping on different traction current sections. By opening and closing certain circuit breakers in the sub stations, and using the readings on their respective equipment, it's possible to trace an earth fault to a particular traction current section.

As for 'current rail gaps that are permanently coupled (ie. no switch)' - surely it's not a rail gap if it's permanently coupled with no switch?!!

Whilst I wouldn't profess to know what the ideal length is, I wouldn't imagine expansion gaps are long enough to qualify as useful for anything - except perhaps a section switch gap at a push.