the UK, only a small percentage of trains (Great Western Railway and Chiltern Railways) are fitted with this equipment. Driver's reminder appliance The DRA is an inhibiting switch located on the driver's desk of United Kingdom passenger trains designed specifically to prevent 'starting away SPADs'. The driver is required to operate the DRA whenever the train is brought to a stand, either after passing a signal displaying caution or at a signal displaying danger. Once applied, the DRA displays a red light and prevents traction power from being taken until the DRA is manually cancelled by the driver. Collision prevention systems Whilst the ideal safety system would prevent a SPAD from occurring, most equipment in current use does not stop the train before it has passed the Danger signal. However, provided that the train stops within the designated overlap beyond that signal, a collision should not occur. Train stops On the London Underground (for example), mechanical train stops are fitted beside the track at signals to stop a train, should an S.P.A.D occur. Train stops are also installed on main line railways in places where tripcock equipped trains run in extensive tunnels, such as the on the Northern City Line where the Automatic warning system and Train Protection & Warning System are not fitted. Automatic Warning System On the UK mainline, AWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and magnets located in the center of the track. At each AWS site, a permanent magnet arms the system and an electromagnet connected to the green signal lamp disarms the system and a confirming chime is provided to the driver. If the receiver does not disarm within one second after arming, a warning tone sounds at the driver's desk and if it is not cancelled by the driver, the emergency brakes will be activated. A visual indication remains set to remind the driver that they have passed a restrictive signal aspect. Train Protection & Warning System On the UK mainline, TPWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and radio frequency transmitter loops located on the track. The 'Overspeed Sensor System' pair of loops is located on the approach to the signal, and will activate the train's emergency brake if it approaches faster than the 'trigger speed' when the signal is at danger. The 'Train Stop System'