Keith wrote:..... Actually, we have a fairly near equivalent with ground frames. These can either be released remotely from the controlling signal box or using a key which the traincrew take with them. The majority have disappeared now, mainly because of loss of wagon-load traffic and closure of the sidings, but the few remaining sidings (plus new ones installed as wagon-load has been on the increase again) these days tend to be controlled directly from the modern signalling centres, where these have replaced the more traditional signal boxes.
...Not to forget a third way of releasing ground frames, although I fear all may now be extinct :
This was the LNER/BR(ER) 'in-section' GF in Track Circuit Block sections of automatic signals, where the GFs (often quite extensive ; e.g., I believe the master* GF at Rayleigh had at least 11 levers), had individual control levers for each signal protecting the connections that it controlled (directly or indirectly*).
For those unfamiliar, I understand that their operation was :
That they had no remote release of any kind, and the signal levers could be moved from their normal (usual) Reverse position to the backlock position, replacing their signal if 'Off' to Danger, at any time.
The track circuiting in the running lines, the protecting signals' approaches, and the point connections, was quite comprehensive.
The signal lever backlocks would only release, allowing the lever to go fully normal, when :
1. either the signals were free of approach locking (no approaching train on the approach side of any signal whose aspect would step down to a more restrictive one), or
2. any train within that distance had been proven by TC occupation/timers to have stopped on the approach side of the signal, or
3. an arriving train needing to work sidings or cross to the other line, had arrived under clear signals to the right position ready for the next move, and the correct combination of TCs was either clear, or occupied long enough to show that the train was at a stand in a correct place, waiting to shunt.
I think GF-operated shunting signals were also provided where necessary.
Once the moves were finshed, and all the GF connections were restored to normal and were not to be used further, the protecting signal levers could be Reversed, and the signals assumed the correct aspects according to TCs being occupied or clear.
*At some places (I believe former GER-route locations at Billericay, Hockley and Rayleigh were all examples), there was more than one GF; one being the 'master' and having the protecting main signal control levers, the other/s being what might be considered 'subsidiary/ies'. Once the master GF was free for use in one or both directions, it could in turn give an appropriate release to a 'subsidiary' GF.
One such case was, I was told, at Billericay, where the main GF was at the London end of the station and included a trailing crossover amongst its functions, while there was a sub-GF beyond the country end of the platforms, working a second trailing crossover.
Ironically, I believe I recall that latter day rationalisations, loss of sidings and yards for example, and perhaps the dislike of points on curves, had some odd end results, with the pre-1990s resignalling remnants at Billericay consisting of the master GF retaining only its main signal control levers, plus a release for the country end GF - whose crossover was the sole remaining pointwork around the whole station area.
Addtl. info. added 11/09/08:
The 'master' GFs (at least) were blessed with a limited illuminated diagram : I think I recall from briefly seeing Rayleigh's one day, that this featured TCs in the immediate area, and indications of when the signal lever backlocks were free for the lever to be placed fully Normal.
Rayleigh's GF was in the open with the diagram 'hooded'. I think the 'master' at Billericay was in a hut.
On re-thinking, I reckon Rochford also had this type of GF(s) arrangements, and that was where it had been mentioned there having been an 11(+?) lever 'master' GF.