The South Yorkshireman - 3/11/19

Nope. We'll beat that on Saturday. Just north of Abergavenny, Pontrilas Tunnel is only 37 yards short.

 

A tunnel is under the ground, the presence of any man made construction on the ground above is irrelevant.

 

Or if there was no way round a steep sided valley and the navvys had no choice but to tunnel under the hill or mountain such as the Diggle and stanedge tunnels what the L.N.W. built, or such as tunnels like Bolsover (now infilled by the coal board) had coal mine workings running under the trackbed and caused subsidence of the internal brick work and as a consequence the dukeries from langwith jct to chesterfield closed in the 1950,s.

 

So what about "cut and cover" lines like much of the Circle, District and Metropolitan lines? Are they just very long bridges?

 

I think that they are tunnels, it is only the method of construction that is different from the usual boring method - no jokes about tunnelling being really boring! If they were bridges they would be short but extremely wide.

Picking up on RalphW's earlier post #104, many year's ago I went to an illustrated talk on the East Lancs Railway given by John Marshall where, in response to a question, he went around the same convolutions about the difference between tunnels and bridges. He went on to cite where Blackburn Rd at Rishton crosses over the line. The line at this point is in a very shallow cutting, the line and road are both straight and meet at a very acute angle. A bridge over the railway would have to be very long (because of the narrow angle). The technology of the day meant that this was impractical/uneconomic. The solution was to build a short tunnel, Rishton Tunnel (68yds) with the road base directly on the tunnel roof. The surrounding land was infilled/raised slightly to cover the tunnel. The road now gently rises from both directions as it passes over the tunnel, there is nothing obvious from the road to show that the railway is there; low stone walls that look like field boundary walls on each side of the road are staggered by about 50yds. Look over the walls and you can't see the tunnel portals. On the train it looks like a tunnel. The tunnel's only purpose is to allow the road and railway to cross; a tunnel cum bridge.

 

But if the metric is time rather than distance, Askham Tunnel near Retford is traversed in 1 second ( 57 yards : 125mph).