> Both metros should have barrier between platforms and tracks since they know where the doors will open.

This only works for environments with one type of rolling stock only. It's not uncommon that you take a ride in a decades-old train (e.g. the oldest trains in the Munich subway hail from 1971, they are expected to run for essentially over 50 years!), switch lines at a station and end up in a train that rolled off the factory line 2 years ago. Each new train generation has, for example, different door widths, door positions, even the number of doors per wagon can change.