Signals changing from green to red

In the running rails there are insulated block joints (IBJs) just after signals, which separate the signaling sections. Simply enough, as the front axle passes over the associated IBJ, then the signal will return to danger. In the case of an automatic signal, when the rear axle of a train passes over the IBJ at the end of the signaling section, then signal turns green again. With semi-automatic signals, the signaler can then clear them after the rear axle is out of the section.

Works similarly for repeater signals, only they use replacement block joint (RBJs) instead of IBJs.

A IBJ (Insulated Block Joint) is just what the bit of plastic is called. It's used to "break" the track up into Track Circuits

A RBJ (Replacement Block Joint) is what it simply it's purpose (to replace a signal to danger after the passage of a train) and is excatly the same physically as any IBJ!

Now I can't remember how it works on the Bakerloo and other lines where they dont have Track Circuits!

Exactly!! ;D ;D ;D ;D

It's like they tell you all the stuff up the "school" and you go out and it's different on each line too ;D ;D ;D

As everyone regurgitates little bits of what they learned about "the simple track circuit" ;D ;D

Well guys I don't think anyone here is 100%!

Where do I start? Firstly there are track circuits on all LU lines including the Bakerloo which has what are known as 'jointless' tracks. Some other lines have jointless tracks too.

Blockjoints come in several types including the Permali, the 666 Mk1, 666 Mk2, Plastic Bonded and the now very popular Benkler to name the more popular types but they all do exactly the same job.

A blockjoint is a blockjoint no matter where it is used and it is the way in which tracks are divided into track circuit sections on capacitor fed track circuits which are what are predominantly used across LU although this is changing and the Jubilee uses more than one type. The division in jointless tracks is achieved by the use of different frequency sensitive circuits on adjacent 'tracks'.

Each signal has what is known as a 'Limit of control' and an 'overlap'. The 'overlap' is the calculated distance required for the train to stop in an emergency if it should pass a signal at danger. The 'Limit of control' is the distance from a signal up to and including the overlap of the next signal. Thus there will always be at least two signals at danger between any two trains.

The track circuits in the 'Limit of control' each have a contact in the 'selection' of the signal relay, such that when all tracks in the selection are unoccupied and when the signal is selected it will energise the signal relay, lower the trainstop by operating a valve and passing air to overcome the return spring and also clear the signal.

In an automatic area the signal will automatically be selected when the trainstop has been proved to have come to the fully 'ON' position following the passage of the previous train. In a controlled area, in addition, the signal must also be selected by a signalman or by a programme machine or computer according to the timetable.

When a train passes a clear signal and 'drops' onto the replacing track circuit it breaks the selection causing the air which keeps the trainstop lowered to be expelled, thus allowing the arm to spring to the 'ON' position. The signal relay releasing also extinguishes the green and illuminates the red.

The trainstop is proved to have come to the fully 'ON' position by a proving circuit which requires
the replacing track circuit to be unoccupied and the trainstop cannot be lowered again before this circuit has been completed. Should the trainstop fail to come to the fully 'ON' position then the replacing track will remain down as if occupied and prevent subsequent clearing of the signal should a track circuit or trainstop failure occur.

The above is the basis of both semi-automatic and automatic simple signal circuitry.

As the dual aspect is mentioned let me expalin that there are two paths to light the red, the normal path which relies upon the signal relay releasing and the alternate path relying upon the trainstop being in the 'ON' position thus if the signal is green and the trainstop valve fails or the air hose ruptures, the motor fails etc the red will be lit by the trainstop while the green remains lit giving a dual aspect.

The humble track circuit in whatever form, there are several types in use, and the basic signal selection circuit and are the building blocks of the entire line signalling system. Trainstops stood the test of time as they continue so to do in the safe and effective control of trains not just at LU but on other railways around the world such as the NYC Subway. These days as with all things there are new ways to ensure safety and the trainstop is being phased out but I think they'll be around for a few years yet.

There are books about railway signalling but LU signalling is rarely covered in any detail or at all, the only real source being the IRSE but the best place to learn is by getting a job in the industry.


Brian

There are two textbooks that directly refer to LU signalling, both have been published as IRSE 'green books' - rather than the 1980 book 'Railway Signalling' and its companion 'Railway Control Systems'.

Both were published as part of a series on 'Route Control Systems' - I think the original was No 21 and the reprint was No 29. (There is someone asleep in my library at the moment so I can't check the references) - both these books have been republished in a combined volume recently.

Sadly these books only really cover programme machine working and very basic 'V' frame interlocking (21 programme only, 29 computer control of same), they won't teach you the basics of trainstop checking relays and things like the 'alternate red' as quite rightly mentioned by Brian - the best thing to do is to check out Tubeprune's [1] excellent site and most of your signalling queries will be dealt with there. Of course you can ask silly questions here, as I do. ;D

[1] www.trainweb.org/tubeprune/signalling.htm for starters.

Thanks guys - that's really helpful
I like Tube prune's site - very clear

That was more or less the path I took. And I still miss the adrenaline rushes..!

He doesn't actually say that but what he says is open to be incorrectly interpreted, the actual quote is;

"....... and secondly a narrow wavy band of stainless steel is welded to the head of the rails. This ensures that rust does not prevent electrical continuity being maintained. "

Obviously the idea of the stainless steel is that it does not rust and it is true to say that there is a better contact between the train wheels and the stainless steel strip than there would be between the wheels and the crown of the rail because the same force is exerted on a smaller cross sectional area.

As we know theory and practice are often incongruent and stainless still wavy strips may not rust but they do oxidise just like other metals and as we also know tracks will 'pickup' quite easily under a stabled train after just a few nights. South Harrow and Acton Town are two places in my old maintenance patch where we had track fuses pulled normally to ensure siding tracks were down and stayed down, a lineman's attendance being required to move trains in/out of South Harrow and Acton Works.

When the 83 stock were being moved from South Harrow to Acton Works it was a dash to South Harrow IMR by van to get a train out of the sidings then a dash back to Acton Town East IMR to put the train into the Works before shuffling off to do my regular maintenance somewhere else .

For those who may not know or don't even realise it new rails are notorious for problems with track shunting in some cases although the weight and friction of the train wheels is eventually enough to shift the rust and special procedures ensure the integrity of the signalling system in such circumstances. It is not unusual to have to take a shovel to the railhead during rerailing to shift the rust.

He does in Principles of London Underground Operations. The photo caption (page 131) reads "...The rail nearest the camera has a wire welded to its top, to ensure that any train using it activates the track circuits through the increased pressure exerted on the rail."