Automatic Warning System: magnets on the railway

I spotted this from the railway bridge at Kennington near Oxford. We guessed that it was to do with signals and would be magnetic. I thought it would be a new thing but they have been around for decades. I realise that I have seen them before but that I've never seen a clean, new one. It is an Automatic Warning System AWS ramp. There is a permanent magnet in it which sets up the train's emergency brake system when the train rolls over. I can't find out whether the train detects a magnetic pulse in an electric circuit or whether the magnet works a mechanical switch. There is also an electromagnet which has the opposite polarity. If this is working, it deactivates the emergency brake system. There is human control over the system - the driver has to acknowledge the pulses. If the signal is a stop signal or similar warning, the current to the electromagnet is turned off which means that the emergency braking system is not deactivated. The really clever bit is that the system is set so that if there is a power cut, the emergency brakes will not be deactivated. It is fail-safe. So that's why one magnet is permanent and the other is an electromagnet. https://en.wikipedia.org/wiki/Automatic_Warning_System