cab - they are not suitable for use with loose coaching stock or wagon maintenance, where vehicle movements are undertaken by a propelling shunting loco (in this case the lead vehicles would not be equipped with the relevant TPWS safety equipment), nor will it prevent a run-away vehicle from entering a protected work area. Variations Certain signals may have multiple OSSes fitted. Alternatively, usually due to low line speeds, an OSS may not be fitted. An example of this is a terminal station platform starting signal. An OSS on its own may be used to protect a permanent speed restriction, or buffer stop. Although loops are standard, buffer stops may be fitted with 'mini loops', due to the very low approach speed, usually 10 mph. When buffer stops were originally fitted with TPWS using standard loops there were many instances of false applications, causing delays whilst it reset, with trains potentially blocking the station throat, plus the risk of passengers standing to alight being thrown over by the sudden braking. This problem arose when a train passed over the arming loop so slowly that it was still detected by the train's receiver after the on-board timer had completed its cycle. The timer would reset and begin timing again, and the trigger loop then being detected within this second timing cycle would lead to a false intervention. As a temporary solution, drivers were instructed to pass the buffer stop OSSs at 5 mph, eliminating the problem, but meaning that trains no longer had the momentum to roll to the normal stopping point and requiring drivers to apply power beyond the OSS, just a short distance from the buffers, arguably making a buffer stop collision more likely than before TPWS was fitted. The redesigned 'mini loops', roughly a third the length of the standard ones, eliminate this problem, although due to the low speed and low margin, buffer stop OSSs are still a major cause of TPWS trips. Recent applications in the UK have, in conjunction with advanced SPAD protection techniques, used TPWS with outer home signals that protect converging junctions with a higher than average risk by controlling the speed of an approaching train an extra signal section in rear of the junction. If this fails the resultant TPWS application of brakes will stop the train before the point of conflict is reached. This system is referred to as TPWS OS