Right, I'll put my former professional hat on. The bars on the inside of rails at both Kings X and Euston are depression bars which come in two fundamental forms, mechanical and electrical.
Mechanical depression bars date I suspect from the era of the Regulation of the Railways Act 1879 which mandated interlocking. They were often used as fouling bars at the entrance to loops to ensure the tail of a train was clear of the fouling point allowing the points to be reversed and the movement in the opposing direction to depart. The GWR seemed to be particularly fond of them, I am not going to try a list of railways who used them. Whilst they obviously worked they are somewhat maintenance labour intensive and the various mechanical linkages and locking interactions meant that they fell out of use as soon as reliable electrical supplies were available. The FPL bar is similar in that it can't be operated if a train is standing on it, but is really provided for a subtly different purpose. It is to hold the route such that even if the signalman returns the signal to danger rapidly the position of the train prevents the signalman from unbolting and reversing the points under the train. It only occurs at facing points, hence it's name, and stems from the Victorian horror of facing points much disliked by again the GW and the Midland, amongst others no doubt. The risk at trailing points was always thought and probably rightly so less severe, although damage to equipment is never inexpensive. I think I'm right in saying that there are no FPL bars left on Network Rail they all having been replaced by track circuits, but they still exist on heritage railways.
Electrical fouling bars (EFBs) are essentially the same thing, the mechanical bit being limted to a short drive to a circuit controller, a switch in a cast iron box about 6" by 4", usually in the 4 ft which can be used to detect the absence of vehicles. It is important to note the distinction, EFBs and track circuits do not detect trains but the absence of them. It is apparently only the inverse of logic but is critical in safety critical circuits. The pics at Euston show that they can be quite long, in essence made up of as many standard units as necessary and electrically linked. Since they are mechanically sort of balanced and only used in low speed applications they're pretty robust and I don't believe they traditionally had damping mechanisms, if so probably just a big spring.
The engine release crossovers at Euston were electrically released by the signalman, the fouling bars play no part, and that is what the point rodding is. The bars detected standing vehicels which impacted on the displayed signal given to a driver of an incoming train. Modern day operation would be a single yellow for an unoccupied platform and a subsidiary two white lights for an partially occupied road.
The signalling installation at Kings X was by SGE and used electric point motors with separate detection and mechanical FPLs. Looking at the pic of the DMU the large box to the right of the slip is the motor and from it a rod is coming towards the viewer to a crank, it then goes right under the slip and the nearest rail of the next track to another crank, the rod goes away to another crank which transfers the drive back to a crank immediately in front of the slip switches where it then drives the plunger of the FPL. From the FPL escapement drive there is rod going to another box that detects the FPL plunger is in releasing signals as required. That's 4 cranks for loss of drive to occur, detection to be lost as coach screws work in decaying timbers, iultiplied by however many times in a horribly cramped layout that was the old Kings Cross, I applied for a job with the Kings Cross DS&TE which I unfortunately didn't get which was a shame as the ER in the 70s was infinitely more interesting that the shiny new place up Euston Road where I went instead.
Regards
Martin
