Not tough to do at all. You simply must anticipate where the stop, or the speed reduction must be achieved by, and place an indication magnet at least braking distance back - same as for approach indications on any kind of system.

Placing the track maqnet at the first (and subsequent) yellow or red signals in approach to the stopping point, does that job easily, as it is the signal's aspect which controls the magnet anyway. BTW, the track magnet is a passive magnetic device with a coil. No energy is ever applied to it, but a signal contact closes on the coil, shorting it out, thus changing the track magnet's "magnetic reluctance"; which difference the train's pickup shoes can detect.

Profoundly simple and elegant actually. Unfortunately, even though the engineer must respond or be shut down, there is only one chance to test for that. And if he Subsequently forgets a restrictive indication, the party is over! Intermittent indications of any sort, including the mere passing of a visible signal; always requires that someone or something must remember the restrictive aspect, in order for it to be effective. And that's where the failures happen; and whether it be a machine's failure to remember, or a human's failure; makes very little difference to those killed or injured in an accident.

This is why ATS (intermittent inductive "Automatic Train Stop") fell out of favor as soon as a better way was developed (ATC, ACS, etc). The better way was to continuously re-read signal indications that pass inductively from the rails. Much better and far more reliable, as the safety of movement is re-calculated continuously, always with fresh data - no need to remember anything.

Too bad PTS, just the same as its oldest brother, will re-introduce a certain amount of intermittent functionality into train control systems, making them less safe. An unmanageable, complicated and inelegant step backwards in my view. The free market did not choose this at all - and neither did responsible regulators, who if they had had the authority, would have fostered the needed improvements to ACS and ATC, requiring that instead. But NO! What else can one expect when bought-off politicians get involved - except technological regression.