First of all, there are numerous studies that show that barriers to suicide are extraordinarily effective at preventing suicide - the vast majority of people who commit suicide do so on a whim, and things like barriers in subway stations or on top of tall buildings, foul smelling gas and others drastically reduce the overall number of suicides in large areas, not only in the specific places they guard.

Secondly, in many studies of attempted suicides, the vast majority of people of people who survived one suicide attempt did not try again (even though the risk of further attempts is much higher with an initial attempt than in the general population, it is still quite low - around 13% in one 35-year long follow-up study).

This doesn't even count the huge number of people that have suicidal thoughts and don't actually attempt suicide - at least sometimes because of barriers like those mentioned before, which prevent a suicidal thought from turning into a rash decision like jumping in from the of the train or off the building.

In fact, for many people, this type of "near-miss" offered by such barriers is exactly what allows them to realize they need urgent help: they don't want to die, but had been ignoring similar thoughts as meer fancies, until such a moment when they realize that they would have actually done and may try again if they don't seek immediate help.