The standard deep tube station is two tubes side by side, each with a platform along one wall. Passengers for one direction never share space with passengers for the other, and each platform can be widened, in principle, without touching the other.
Clapham Common is not that. It has a single island platform in one wide tunnel, with tracks on both sides, which means everyone waiting for either direction stands on the same strip of floor.
Why it was built that way
Building one large tunnel instead of two smaller ones can be cheaper where the ground allows it, and it puts the stairs and lifts in one place rather than two. On paper it is an efficient arrangement.
What the paper does not show is what happens when both platforms are busy at once. The island has to hold two crowds, and they face in opposite directions, and the space between them is the space people walk through.
What it looks like now
The arch in the photograph is the giveaway: a single vault rather than two, with the platform running down the middle of it and the tracks tucked under the curve on either side.
Stations built later went back to two tunnels. The ones built this way are still in service, and at the busiest hours staff manage the platform rather than the trains.
