Talk:Combe Down Tunnel

The slope

The tunnel is described as "climbing towards Bath" but this means that the line is climbing away from its summit toward the River Avon although it crosses the latter on a fairly low bridge. Ir also has the accidental runaway mentioned doing so in uphill. I suspect it is wrong. Britmax 18:30, 11 November 2007 (UTC)

The summit (of this section of the line) is just at the north entrance to the tunnel, so the article is right. Trains were often banked up from Bath to the tunnel entrance; the banking engines then reversed back down the slope into Bath while the train proceeded downhill inside the tunnel and then down the valley to Midford. The slope northbound from Midford to and through the tunnel wasn't steep enough to require banking. Johnlp 18:46, 11 November 2007 (UTC)

Gradient ?

What is Gradient of of tunnel, which AFAIK climbs away from Bath? Tabletop (talk) 05:09, 8 October 2011 (UTC)

There is a cross section diagram here but I don't know how to calculate the gradient from it. On Forgotten relics article Combe Down Tunnel it says "the predominantly dry tunnel rises on a 1 in 100 gradient before leveling out around 400 yards from the northern portal." Is this what you were asking & should we put it in the article?— Rod ^talk 09:06, 8 October 2011 (UTC)\\

Calcs

The short tunnel say 800m @ 1 in 50 = 16m fall. See Devonshire Tunnel.

.

The long tunnel is slightly better than 1 in 100, say 1 in 110, because part of it is level. It is a little difficult to read the lengths off the graph.Tabletop (talk) 14:36, 30 April 2013 (UTC)

Long tunnel 1272m @ 1 in 100 = 12.72m rise

Long tunnel 0400m @ 1 in Lvl = 00.00m rise

Long tunnel 1672m @ 1 in 131 - 12.72m rise (average gradient)

Why was the long tunnel built with two gradients, 1 in 100 and level, instead of a more gentle 1 in 131 average ??

Because the long tunnel pierces a summit, if it were made a bit longer, it could have been made entirely level (or very gentle say 1 in 400 to drainage purposes).

Because the long tunnel is approached by 1 in 50 gradients, if the approach gradient were made a bit steeper, or with higher embankments, and the southern entrance made higher, the gradient in that 1 in 100 tunnel could have been made easier or even entirely level. Better to stall on say 1 in 45 in the open air, than in any tunnel. Tabletop (talk) 02:53, 1 May 2013 (UTC)

Bank Engines

"This operation was a very rare example of two trains being permitted to run within a single-line section ...."

Since the train and the bank engines, when separated, operate in opposite directions, there is no risk of a collision, and therefore the above quote about "two trains" is misleading.

When two trains are allowed in a section in the same direction, whether double or single line, mostly only goods trains, this is called "permissive working".

See also

• Ruling gradient Tabletop (talk) 03:05, 1 May 2013 (UTC)
• Devonshire Tunnel on other side of summit.

Fume problems??

Since the 1 in 100 Combe Down Tunnel was half the ruling gradient of 1 in 50, one might expect that fume problems would not always occur.

Other factors:

• Tunnel gradient relative to Ruling gradient of line
• Wind;
• Wet rails,
• Momentum from the approaches
• Difference between Structure gauge and Loading gauge
• Compensation of curves for sharpness
• Intermediate ventilation if level of surface above tunnel is small.
• Double or Single track tunnel
• Escape routes for crew when operating double headers or Garratts.
• Availability of sidings to stow waggons and reduce the load of heaviest trains should train fail.

Exactly how gentle the tunnel gradient would need to be guarantee no fume problems is hard to say. Tabletop (talk) 03:13, 1 May 2013 (UTC)

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Portal?

That 'The tunnel portal in 2013' photo isn't a shot of Combe Down Tunnel.. Dave F63 (talk) 11:31, 19 January 2023 (UTC)