How to design a metro train stop that would prevent people from endangering themselves by accessing the rails when a metro arrives (perhaps by implementing the design of special doors or elevators leading into the train itself)?
Turin's unmanned tube (Italy) has barriers that extend from the quay to the ceiling of the basement. It is virtually impossible to fall on the tracks without tampering with the automatic doors.
I think this is a great solution.
You can't design a sign that stops people from doing that, you would need something physical which makes it impossible to enter the rails, just like in cities like singapur, where a gate is placed in front of the rails which only opens when the train opened it doors so people can enter.
What you can do is place something like a printed line 1 meter before the rails on the ground with flashy colors like yellow/red/pink etc. and place a text like "Watch your step" etc. between it.
Bonus points if you don't just use a printed line but use a physical line that has a different texture/height then the ground, people will automatically be aware that they are close to the rails when stepping on that.
Found an example on google which shows you what i mean
london Westminster tube stop has physical barriers that make it impossible to cross onto the railway. Double doors synchronise with the train doors on arrival, like the double doors on a lift.