What is Track Circuiting in Railway Signalling System?

Question

What is Track Circuiting in Railway Signalling System?

Answer 1

Assuming you already know a bit about signalling; the running rail is dividing into a number of sections and a current is fed through each section to its own relay – this is the basis of a track circuit. The track circuits are then kept isolated from each other along the running rail by the means of an IBJ (insulated block joint).

A section is one part of a track that a signal will control. In between signals, there may be several sections, each with its own track circuit. The track circuits will usually be named after the signal that is controlling that part of the track. So let's say you've got a signal named A123, its track circuits may be 123A, 123B and 123C (for each of the sections ahead of that signal). If any of the track circuits are occupied by a train (and this is done by the contact the train makes with the rails), then the signal will not clear because the track circuits let the signal show that the sections ahead are occupied.

It gets more complicated (stuff about signal overlaps, etc.) and all railway signalling systems have their own variations, but the basic principles of modern railway signalling are usually the same. I can't really explain it any other way!

Answer 2

I believe this means an energized track, for crossing protection, and train signaling systems. A small amount of electricity flows through both rails. As a train approaches, the electricity in each rail becomes bridged when it crosses the wheels and axles of the train. This shorts out the system and activates the electric device - crossing gates, signals, etcetera. The distance the electricity flows through the rails on either side of the electric device is determined by the placement of insulated rail joints. These non conductive joints prevent the entire railroad from being energized. Like I said, it is a very small amount of electricity. Probably something close to the output of a flashlight battery.

Answer 3

The two running rails carry a low voltage current which is supplied from trackside signalling boxes. Quite often these days it is an AC supply.

This track-circuit current is carefully separated by cleverly designed lineside and train-borne circuitry from any traction current.

The lengths of track in a "circuit section" are separated by insulated joints at the butt end of the rails. If you look closely at rail joints you can sometimes see them. Where lengths of track are in the same "circuit section " you will see pairs of thin wires bridging the rail joints.

A train in the section will complete the circuit by short circuiting the rails through it's steel wheels and axles. This means that a (track circuit) current will flow which will be detected by the lineside signalling equipment.

That way the system knows that a train is present and it will show up on the control panel of the controlling signal box and the appropriate automatic signal lights/crossing gates/train protection systems etc. will be operated.

Track cicuiting is pretty well universal on all British railway lines and most European ones (all main lines are equipped).

The system was developed in Victorian times as a means of indicating to signal boxes when a train was held at a Danger (stop) signal at night or in fog. They seemed to have more fog in those days than we do now.

The presence of a train caused the track circuit (DC in those days) current to flow and this caused an electromechanical relay to operate and prevent the signalman from allowing any following train to come into conflict with the stopped one.

These days the track circuit is responsible for automatically setting signals to Red just after a train has passed it. When the system detects that the train is past the following signal the first one will turn yellow (or double yellow in a four-aspect area) followed by green (or yellow in a four aspect area) as the train enters the next section along again... and so on.

You could spend hours happily sat by a four aspect signal watching it change colour as trains pass. It's more exciting than angling or golf.

Track circuiting  was such a good idea that all UK railways quickly adopted it and it spread beyond local use (just in front of signals) to being used along whole lengths of line. The continentals, Russians and USA copied the idea. Now there are variants of the original idea all over the civilised world.

Answer 4

Hi there.

Track circuits are use to detect where a train is on the line. There is a small current that is sent through the rails, which when a train runs along completes a ciruit, so turning the signal directly behind it. Once the train moves onto the next pice of track circuit, the signals that were red go to amber, and the signals that was amber turn to green.

I know that this answer is a very simple, and there is alot more to it. But I hope that my answer helps.

Answer 5

Railroad track is steel, so each rail conducts electricity.  The two rails don't naturally connect to each other.

A railroad axle (wheel + axle + wheel) is a solid piece of metal, so it conducts electricity between the two rails.

As you can imagine, this gives all sorts of electrical ways to detect the presence of trains.  That is the essence of a track circuit.

It's one of the ways they keep trains from hitting each other.  As such that's not something to be seen asking a lot about, "after 9/11" as us Americans say.  As if that were anything new.