Philosophical question: Tunnels

[RJSRdg]

When is a tunnel not a tunnel?

I'm sure we would all regard the Tube tunnels that were dug with the Greathead Shield or with a TBM as tunnels.
Likewise, the cut-and-cover parts of the sub-surface lines were dug out through something that already existed, so are tunnels.

However, I would not regard the road 'tunnels' underneath Waterloo and London Bridge stations as 'tunnels' - they are in fact very wide overbridges, built over roads that already existed.

BUT - what about the parts of the District Line underneath the Victoria Embankment? For that stretch, the wall alongside the river was built first, reclaiming the land on which the District Line was then built. The roadway was then built over the top. So it's not really a tunnel. as nothing was dug, and it's not really a bridge, because the road runs in the same direction as the railway rather than across it. So, what is it?

[RobbieM]

It's a question I've pondered at times, though not long enough to come up with an answer. But I agree the District Line along the embankment shouldn't actually be called a tunnel. Perhaps the road/rail combination is a double-decker transport corridor?

[DavidC]

Uncut-and-cover?

[Lunaticonthegrass]

I'd call the District line at that point an artificial tunnel.

I'm tempted to say that a bridge principally has a means of transport over it, whereas a tunnel has something else over it. However, that would make that section of the district line a bridge, which I don't believe it is. Bridges cross whatever's beneath them directly, usually using the shortest route possible i.e. straight from one side to the other. Tunnels don't do this, they do something else, whatever that may be. So the District line in the Embankment is in a tunnel.