[Retric]

Trains travel in each direction so 20 trains per direction or 6%, unless this is entirely single tracked.

We can also exclude any car stopped at a station, as you can have multiple cars unloading at the same station or have a train bypass a station without making a wider tunnel.

[spiralx]

I included both directions in my calculation, hence the denominator being (2 * 21km). And for the Underground there are no passing trains or multiple trains unloading at a station, platforms are generally only a few metres longer than the train itself.