The driver can't usefully brake in response to a person on the track, but they do need to be able to brake in response to a trackside signal. At 140mph, braking

The driver can't usefully brake in response to a person on the track, but they do need to be able to brake in response to a trackside signal. At 140mph, braking distances are greater than sighting distances - by the time you've seen a signal, it's too late to stop. That's catastrophic in a fixed-block signalling system, which can only detect the presence or absence of a train in a particular signalling block; if you end up with two trains in the same block, you've got to run the whole line at a crawl to restore safe separation distances between trains. High speed trains universally use moving-block signalling with in-cab signal indication; rather than relying on fixed trackside signal lights, the signalling system constantly tracks the exact location of each train, creates a virtual signal block around that train and indicates signal aspects and line speeds to the driver on a display in the driving cab. https://en.wikipedia.org/wiki/Railway_signalling#Fixed_block https://en.wikipedia.org/wiki/Railway_signalling#Fixed_block