Improvements in AWS

An electromagnet for a railway advance warning system (AWS) comprises a switch (16) configured to modulate a supply voltage to an inductor coil (12) with pulse width modulation (PWM) signals. A freewheeling diode (flyback diode) (15) is connected in a loop with the coil (12) and configured to be reverse-biased against the supply voltage when the switch (16) is closed, and forward biased relative to the coil (12) when the switch (16) is open. A current sensor (13) is configured to detect current through the coil (12) and a controller (14) is configured to control the current through the coil (12) based upon the sensor (12) reading. The controller may comprise a comparator and a waveform generator. The switch may be configured to modulate the supply voltage at a switching frequency of 1 kHz to 10 kHz. In a further aspect of the invention, the electromagnet forms part of a suppressor magnet (10), which also comprises a permanent magnet (11) made of samarium cobalt (SmCo).