Disclaimer: I'm no expert but this is how I would approach it.

1. Ask for or Estimate(pick a easy number to calc) the number of train routes in London

2. Assume average number of stations in a route - pick number

3. Assume average time between stations - this is also the frequency of trains at a station

4. State that for ease of calculation, you assume that at any point in time there is only train between 2 stations in any direction

5. No of trains = multiply Routes from 1 * (Avg No of stations -1) * Avg time taken between stations * 2

Multiple by 2 because train routes are bi directional

For extra point - say there would be a buffer of 10% for maintenance/peak times

or you can assume a certain percentage of routes are circular so you wont need to mutiply by 2

Disclaimer: I'm no expert but this is how I would approach it.

1. Ask for or Estimate(pick a easy number to calc) the number of train routes in London

2. Assume average number of stations in a route - pick number

3. Assume average time between stations - this is also the frequency of trains at a station

4. State that for ease of calculation, you assume that at any point in time there is only train between 2 stations in any direction

5. No of trains = multiply Routes from 1 * (Avg No of stations -1) * Avg time taken between stations * 2

Multiple by 2 because train routes are bi directional

For extra point - say there would be a buffer of 10% for maintenance/peak times

or you can assume a certain percentage of routes are circular so you wont need to mutiply by 2