The essential function of signalling is to prevent two trains being on the same bit of track at the same time.  In a few countries, there are operating conditions that allow drivers to "stop and proceed" as a normal procedure, ready to stop behind a proceeding train.  In most countries, danger signals are absolute, meaning that trains cannot proceed unless the signal itself is faulty.

Most high speed railways have colour light signals at the side of the track (the ones that don't have the signalling in the cab).  These are operated by track circuits.  Each section of track is electrically isolated from the next.  It is connected to a power supply, and when the wheels of a train pass over it, the circuit is completed to close a relay.  The wheels act as a switch.

The pictures below show a possible very simple track circuit.

We can see how the wheels act as a switch:

The problem with this is that it is not fail-safe.  If the relay fails, a clear signal could be given even if there is a train on the track.  We need something that will ensure that if the relay fails, a danger signal is shown (a "right-side" failure).

So when there are no wheels on the track, the relay is switched ON and the signal shows a clear aspect.

If we put a train on this section of track, we get:

So the circuit is shorted out by the wheel set and the relay is turned off.  However shorting out batteries is not a good idea:

• The battery will get flat pretty quickly;
• The internal resistance of the battery will heat it up.

We get around this by using a current-limiting resistor in series, as shown in the modified circuit below:

We can show this as a normal electric circuit:

The current limiting resistor's function is:

• To limit the current from the battery when the wheels short out the relay, when the track is occupied;
• To control the voltage across the relay when the track is not occupied.

This circuit acts as a series circuit when the switch is open (i.e. no train in the section).  If you need to revise series circuits from the AS level notes, click on the button:

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It is also a potential divider circuit.

We can say that

 voltage across the relay = the battery voltage - voltage across the current limiting resistor

As soon as the switch is closed, the voltage across the resistor falls to (almost) zero.

To set the value of the current limiting resistor, we need to know the current needed to operate the relay, its resistance, and the source voltage.

There are limitations to the above arrangement which give a false clear:

• Wheels or track are rusty, so that the train is not detected.  Sometimes zig-zag patterns are welded to the top of the rail where this is thought to be a problem.
• Leaves on the line.  The slippery mush formed by crushed leaves offers a high resistance, as well as causing wheels to slip.
• A derailment with all wheels off the track.  All trains carry track-circuit clips to protect the train in such circumstances.

The following problems lead to false danger signals:

• Wet sleepers and ballast (stones between the track) provides alternative pathways leading to the relay being shorted out.
• Near the sea, the salt spray can make this even worse.

The latter situations are like putting a second resistor in parallel to the relay:

If you need to revise parallel circuits from the AS level notes, click on the button:

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If we consider the circuit now, there are four things that will happen:

1. The resistance of the lower half of the potential divider circuit will fall.
2. The current through the current limiting resistor will increase.
3. The voltage drop across the current limiting resistor will increase.
4. The voltage across the relay will fall. 

This will cause the relay coil to lose some of its magnetic field.  It will then release the contacts, and make the signal go red.

Where there are persistent problems with wet track, e.g. in tunnels, axle counters are used.