There are not many/any high-frequency metro systems that can do this on the same rails. At peak times, when you could do with the expresses the most, there can

There are not many/any high-frequency metro systems that can do this on the same rails. At peak times, when you could do with the expresses the most, there can be stopping services 1/2 minutes apart on average, which means non-stoppers are simply not possible. On regional railways, where the service might be every 30 minutes or longer, it is a different prospect since you can send the stopper out immediately after the express and it gets 30 miuntes to get out of the way.