Wednesday, 28 November 2018

A Wet Day

Tuesday 27th Nov

A little progress at Broadway today with the completion of the
installation of the signal wire stakes up to Childswickham Bridge (all now concreted in).
And here is the newly installed advanced starter signal post  adjacent to the bridge which was planted last Friday by JCB et al.

While we had the mixer going, a concrete block has been prepared to mount the casting of the detector unit for the south points:

Now that it is correctly positioned, we have taken a measurement for the length of a further bar to secure the casting to the nearside rail - this to ensure that the detector unit cannot move relative to the track (just in case the concrete block moves!!) Hopefully next week I can show you a picture of the finished set-up.

Last Friday saw the erection of three posts for the home, advanced starter and distant signals. George B captured this picture of the distant being tamped in by the JCB:

Notes about the cabling required between Toddington and Broadway:

So far a 20 pair cable has been trenched in up to Buckland where a line-side connection cabinet is required. This corresponds with the insulated breather joint of the continuously welded track, so connections for track circuiting will also be made here. The advanced party of Neil C and Keith L took the pickup with the necessary aggregate, etc to mix concrete on site. Here are the before and after:

Note the 90 deg pipe to feed the cable up into the cabinet.
This cabinet will also house the batteries and relay for energising the track circuit.
The next length of cable will terminate at the distant signal where another trackside cabinet will be required.
Altogether there will be several more of these cabinets required as the cabling progresses.



  1. I'm surprised that there are going to be batteries in the lineside cabinet for the track circuit. I just assumed that any battery supply would be back in a signal box. What's the reason for putting them in the cabinet?

  2. volt drop over the length of the cable would mean that by the time the supply reached the track feed position you wouldn't have the required voltage. Sealed batteries can last years in this situation, or a small battery charger topping up a battery can also be used, both equally as reliable with the correct maintenance regime.

    1. Ah! Your reply prompted me to take a look at the IRSE document on DC Track Circuits for Minor Railways. I was surprised that sometimes primary cells were used to power the track circuit. I wonder how long they lasted? I expected secondary batteries to be used but there is the problem of charging them in a cabinet. Curly mentions a 20 pair cable but nothing about a mains supply for a charger. Perhaps several wires could be paralleled to provide enough current to keep a local battery topped up from a charger in the signal box? I am assuming that the track circuit normally takes 100 to 200 milliamps which would be possible to feed over telecoms type multicore cable if enough voltage was used at the far end? Trying to understand the technicalities of railway signalling here!

  3. Thanks for the update Curly. Any and all blogs are avidly awaited and enjoyed by all your readers.
    Regards, Paul.

  4. The resistance of the cables we use is approximately 20 ohms/km. The track circuit relays have a pull up voltage of around 0.4V and saturate at 1.5V. Their operating current is a nominal 100mA. Each track ciruit has a 4.5V DC supply provided by two, series connected, 25AH sealed-lead cells which have a working life of 10+ years. A simple electronic regulator trickle charges the cells. The DC supply for the regulator comes from the signal box.


    1. Thanks for the reply, Kevern. So I assume that each wire will carry about 100mA once the battery is being trickle charged and the relay is pulled in (with the track circuit unoccupied). This would give about 4V per km drop (go and return wires). So you could feed, say 5km with a 28V battery supply from the signal box, and still have enough voltage for a simple regulator/charger circuit to work. I suppose alternatively you could use a switching regulator for less current in the wires but still the 100mA into the battery/relay circuit. That would give a line current of less than 20mA by my calculation. So how many track circuits are needed for the Toddington to Broadway line - will each one do about 1km?

  5. Kevern is presently building a large number of these regulators. They will form a standard method of maintaining the track circuit supplies. It is hoped that an article will shortly be written for "The Cornishman"about this system and also the one that monitors these systems at Gotherington.
    This, and a lot of other things, all go on behind the scenes in S&T.

  6. Individual Track Circuits only operate over a finite distance - and certainly not the distance between Broadway and Toddington! Presumably the line will be continuously track circuited between those two boxes in order to allow Acceptance Lever working, so will you be using cut-section tracks or Axle Counters?