It's usually fine for high frequency metros, particularly in peak times with more trains, but time-based zone fares aren't always reliable in other cases. For

It's usually fine for high frequency metros, particularly in peak times with more trains, but time-based zone fares aren't always reliable in other cases. For example, in most of Switzerland and probably Germany that uses unified zone systems for everything (intercity trains, local buses, boats, funiculars, etc.) the frequency of most train routes is nowhere near those of a typical bus.