direction. Since a train may be required to pass a signal at danger during failure etc., the driver has the option to override a TSS, but not an OSS. When a subsidiary signal associated with a main aspect signal is cleared for a shunting movement, the TSS loops are de-energised, but the OSS loops remain active. Where trains are signalled in opposite directions on an individual line it could be possible for an unwarranted TPWS intervention to occur as a train travelled between an OSS arming and either trigger loops that were in fact associated with different signals. To cater for this situation one signal would be nominated the ‘normal direction’ and fitted with ‘ND’ equipment. The other signal would be nominated the ‘opposite direction’ and fitted with ‘OD’ equipment. Opposite direction TPWS transmission frequencies are slightly different, working at 64.75 (OSS arming), 66.75 (TSS arming), and 65.75 kHz (common trigger). Location equipment At the lineside there are two modules associated with each set of loops: a Signal Interface Module (SIM) and an OSS or TSS module. These generate the frequencies for the loops, and prove the loops are intact. They interface with the signalling system. SIM Modules are colour coded red ND TSS Modules are colour coded green OD TSS Modules are colour coded brown ND OSS Modules are colour coded yellow OD OSS Modules are colour coded blue On-train equipment Every traction unit is fitted with a: TPWS receiver. TPWS control panel (standard or enhanced version). AWS/TPWS acknowledgement button. TPWS temporary isolation switch. AWS/TPWS full isolation switch. If the loops are energised, an aerial on the underside of the train picks up the radio frequency signal and passes it to the receiver. A timer measures how long it takes to pass between the arming and trigger loops. This time is used to check the speed, and if it is higher than the TPWS 'set speed', an emergency brake application is initiated. If the train is travelling slower than the TPWS set speed, but then passes the signal at danger, the aerial will receive the signal from the energised Train Stop System loops, and the brake will be applied to stop the train within the overlap. Multiple unit trains have an aerial at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one aerial. This would be either at the front or rear of