Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Model of train track signalling


This activity models a railway track-circuit signalling system. Train wheels and axles provide an electrical connection between the two rails on which they travel. This enables a track signalling circuit to detect the presence of a train on a given section of track.

Students use their existing knowledge of resistors combining in parallel, and of internal resistance, to explain its operation.

Apparatus and materials

Model rail section

Track circuit relay and signals unit

Power supply, 6 V, AC

Variable resistor, 20 ohm, 3 W

Dry cells, 1.5 V, 4

Resistors (rated for 7 W power) from 0.22 ohm to 10 ohm (e.g. Maplin L0.22 - L10R)


Crocodile clips, 2


Metal foil (steel or copper), thick strip about 1 cm x 5 cm

Health & Safety and Technical notes

Read our standard health & safety guidance

See apparatus entries for construction details.


a Connect the units together as shown in the diagram. 

Circuit and train track Photo courtesy of Mike Vetterlein

b Simulate a train on the right-hand section by holding a piece of steel or copper foil across the rails. Observe what happens to the lamps.

c Simulate different ballast conditions by placing various resistors across the rails. (‘Ballast’ is the loose pieces of stone spread between and around the railway sleepers.) Do not leave them in contact for long, as they will get hot and exhaust the battery.

Note the smallest resistance that will cause the signal to switch from green to red.

d Connect a voltmeter across the relay solenoid and connect a 20 Ω variable resistor across the tracks.

e For various settings of the variable resistor, find the drop-away voltage (when the signal switches from green to red) and pick-up voltage (red to green).

After each set of measurements, disconnect the variable resistor and find its resistance using a resistance meter.

f Study the circuit diagrams and explain as precisely as possible how the track signalling circuit works.

Discuss what would happen if the power supply had no internal resistance.


Teaching notes

1 This activity is intended for use with students who are already familiar with the idea of internal resistance and with resistor combinations in parallel. The intention is that they study the behaviour of the circuit and use their existing knowledge to explain its operation. 

A key feature of the circuit is that the power supply has some internal resistance.

The relay is connected across the terminals of the power supply. When a low resistance is connected in parallel with the relay, the net resistance falls. The terminal potential difference falls, and hence the current in the relay also falls.

If the relay current drops below a certain threshold, the switch is released and the signal shows red.

If the power supply’s internal resistance were zero, reducing the resistance in the external circuit would not affect the terminal potential difference. The current in the relay would be unchanged and the signal would permanently show green.

2 It is important to avoid explanations along the lines of ‘most of the current goes along the short circuit and by-passes the relay’. That implies, incorrectly, that the total current remains constant, and is merely divided up differently between the relay and other resistors in parallel with it.

This experiment comes from Salters Horners Advanced Physics © University of York Science Education Group.

Diagrams are reproduced by permission of the copyright holders, Heinemann.

This experiment was safety-checked in June 2007