Do railway lines have any safety mechanisms built in to bring a speeding train to halt or at least altert a signaller of this? I know that when a train approaches yellow/red aspects there are grids in the four foot which check if the train is slowing enough to stop but I wondered if there was anything that would highlight a train travelling too fast on a clear path of green lights? Other than the timings giving it away, if there was a huge stretch of clear line for a few miles and a train in a 125pm zone ran at 130pm or even above as some are capable of, would there be any alerts (to either driver, or signaller)? Would the train be stopped? I'm just interested in the railway equivalent of the roadside speed camera. Not that I am suggesting any driver would do this purposefully, but if there were mitigating circumstances such as illness or simple misjudgment in extreme cases.