that while it provided a relatively cheap stop-gap prior to the widescale introduction of ATP and ERTMS, nothing should impede the installation of the much more capable European Train Control System. How it works Overview A pair of electronic loops are placed 50–450 metres on the approach side of the signal, energized when it is at danger. The distance between the loops determines the minimum speed at which the on board equipment will apply the train's emergency brake. When the train's TPWS receiver passes over the first loop a timer begins to count down. If the second loop is passed before the timer has reached zero, the TPWS will activate. The greater the line speed, the more widely spaced the two loops will be. There is another pair of loops at the signal, also energised when the signal is at danger. These are end to end, and thus will initiate a brake application on a train about to pass a signal at danger regardless of speed. On-track equipment In a standard installation there are two pairs of loops, colloquially referred to as "grids" or "toast racks". Both pairs consist of an 'arming' and a 'trigger' loop. If the signal is at danger the loops will be energised. If the signal is clear, the loops will de-energise. The first pair, the Overspeed Sensor System (OSS), is sited at a position determined by line speed and gradient. The loops are separated by a distance that should not be traversed within less than a pre-determined period of time of about one second if the train is running at a safe speed approaching the signal at danger. The exact timings are 974 milliseconds for passenger trains and 1218 milliseconds for freight trains, determined by equipment on the train. Freight trains use the 1.25 times longer timing because of their different braking characteristics. The first, 'arming', loop emits a frequency of 64.25 kHz. The second, 'trigger', loop has a frequency of 65.25 kHz. The other pair of loops is back to back at the signal, and is called a Train Stop System (TSS). The 'arming' and 'trigger' loops work at 66.25 kHz and 65.25 kHz respectively. The brakes will be applied if the on-train equipment detects both frequencies together after having detected the arming frequency alone. Thus, an energised TSS is effective at any speed, but only if a train passes it in the right