I think most of it is legacy. Trains have been around for a long time, and changing something on tens of thousands of locomotives and millions of train cars wou

I think most of it is legacy. Trains have been around for a long time, and changing something on tens of thousands of locomotives and millions of train cars would be very expensive and complicated. Most systems now have a reserve tank on each car that holds a certain pressure, and the braking signal is sent by reducing the pressure from the main unit. But it's still not nearly as safe as automotive air breaks that fail closed, since eventually all the air will leak out of the tanks. https://en.wikipedia.org/wiki/Railway_air_brake https://en.wikipedia.org/wiki/Railway_air_brake