You just don't, it's not really needed.

Never underestimate the poder of idiots in large amounts.

"hundreds occupied high-speed railway tracks for around 3 hours, blocking services linking Figueres, Girona and Barcelona,"

https://www.thelocal.es/20181001/pro-independence-protesters...

A thick log or dead deer across the tracks of a high speed train could lead to catastrophe.

I once rode the Eurostar from London to Paris and as I recall all the high speed sections were enclosed in very high, barbed wire fence. So some security is definitely needed. Collision energy goes up with the square of velocity.