Hi Mirko,

Below is one potential approach to the solution :

- Number of trains per hour (assume this is based on frequency of service), you can also adjust for time of day (rush hour vs other times vs post 9pm etc.) – for now let's say there are 10 trains per hour

-How long the trains in the subway run (Lets say 16 hours)

-Number of drivers per train (let’s assume 1 for this exercise)

- How long it takes to do one loop – let’s assume 1 hour in each direction and 0 turnaround time

- Duration of shift (let's say 8 hours) – so we need at least 2 drivers for the whole day per train assuming a 16 hour day

Now : 10 trains per hour going each direction, given that the loop takes 2 hours to complete, you need 20 the trains at all times. Each train however needs 2 drivers per day given the shifts – so you would need 40 drivers in a day per line under the assumptions laid out above

Of course you can add many complications to this such as time of day, turnaround times, breaks for drivers etc. that will impact your answer