(Trivia) When does a bridge become a tunnel?

[Howardh]

Stupid question?!

Thinking when you go under motorways, buildings etc the line could be going under lengths greater than some small tunnels. Is there a specific definition of what a tunnel is, how long it has to be to be classified as such? (hash-tag slowmovingday)

 

[carriageline]

On the railway, bridges generally have bridge numbers, tunnels have names! That's all I know

 

[talltim]

When is grows up

 

[driver9000]

Tunnels are in the Sectional Appendix, bridges aren't.

 

[andythebrave]

Something like bridges carry roads, waterways or other manufactured items over a railway and tunnels have natural landscapes above. Or something like that.

 

[EM2]

Bridges go over or under something, tunnels go through. Think verb, not noun

 

[Tio Terry]

There is a definition and I used to know it!

It's to do with Fire, structures over a given length have to have special cable sheaths , ZHLS, (Zero Halogen Low Smoke) to ensure they don't give off too much smoke in tunnels in the case of a fire. I have in the back of my mind 500m but please don't quote me on that, I need to check it out first.

 

[Tomnick]

Tunnels are in the Sectional Appendix, bridges aren't.

This is the definitive answer as far as the operational railway is concerned.

 

[Howardh]

There is a definition and I used to know it!

It's to do with Fire, structures over a given length have to have special cable sheaths , ZHLS, (Zero Halogen Low Smoke) to ensure they don't give off too much smoke in tunnels in the case of a fire. I have in the back of my mind 500m but please don't quote me on that, I need to check it out first.

Longer tunnels would need airshafts, right?
If there was an incident in a short tunnel, it's a possibility that the train could be stuck inside but with one or both ends sticking out, which may affect evacuation proceedures?

 

[Bayum]

Bridges go over or under something, tunnels go through. Think verb, not noun

Pardon?

Verb, not noun?

 

[EM2]

Pardon?

Verb, not noun?

Yes, you can bridge something with a bridge, or tunnel through something with a tunnel.
A mole is wonderful at tunnelling, but I doubt you'll ever see one bridging a thing!

 

[BantamMenace]

Pardon?

Verb, not noun?

Ie what is the section of track in question doing, not what is it called. Is it going through or over or under something as apposed to what is it called.

 

[Tio Terry]

Longer tunnels would need airshafts, right?
If there was an incident in a short tunnel, it's a possibility that the train could be stuck inside but with one or both ends sticking out, which may affect evacuation proceedures?

Not always. North Downs Tunnel on HS1 has no airshafts for example. I need to get to my archives to be able to tell you why a particular distance was used to determine when something became a tunnel and I cant do that until tomorrow. The European Railway Agency define a tunnel as being 100m long meaning something of that length or greater must have a full Risk Assessment and Evacuation Plan.

Evacuation procedures do, of course, vary with each site. Modern tunnels that comply with the Interoperability TSI will have clear walkways down one side of the train as per North Downs and Heathrow. Others wont and rely on end cab evacuation, East London line for example. Signalling systems are usually designed such that trains are not stopped in tunnels - at least modern systems are! But there are still some places where this happens. It's all down to the Risk Assessment and how the Local Fire Authority view the situation.

What I was referring to was the need for ZHLS cabling which must be provided in a "Tunnel" as opposed to a long bridge. All Tunnels have to have that kind of cable and from memory the length is 500m.

 

[Tomnick]

There are certainly plenty of tunnels far shorter than 500m, and indeed I'm fairly sure that there are some tunnels that are shorter than some overbridges are wide - so I remain unconvinced that, for existing structures at least, it's as simple as "longer than x".

 

[Elecman]

i think a bridge becomes a tunnel when it exceeds 20 metres in length

 

[Ploughman]

There is a definition and I used to know it!

It's to do with Fire, structures over a given length have to have special cable sheaths , ZHLS, (Zero Halogen Low Smoke) to ensure they don't give off too much smoke in tunnels in the case of a fire. I have in the back of my mind 500m but please don't quote me on that, I need to check it out first.

So in the days of George Stephenson and IK B were they aware of ZHLS
When they were digging out their tunnels or Bridges?

Length of structure may come into the decision as to which but I think the answer would be closer to 50m than 500m
As said in other posts though there are many Tunnels that are shorter than some bridges.

Anyway this has all been gone through in another thread. I think some time last year.
Getting a bit like Datum plates this one.

 

[HowardGWR]

It's so simple, i am amazed at this discussion. A tunnel is when one goes below earth that has not been disturbed. A cut and cover tunnel is therefore a bridge.

 

[AM9]

It's so simple, i am amazed at this discussion. A tunnel is when one goes below earth that has not been disturbed. A cut and cover tunnel is therefore a bridge.

So most of the District Line, Metropolitan/Circle Line and Thameslink Core runs under bridges?

 

[fgwrich]

Yes, you can bridge something with a bridge, or tunnel through something with a tunnel.
A mole is wonderful at tunnelling, but I doubt you'll ever see one bridging a thing!

Unless it designed the Channel Tunnel Rail Links Bridge/tunnel entrance over the ECML though?

(no external gaps between the actual tunnel mouth and the mouth of the bridge over the ECML - So surely that counts as a Bridge that's also the edge of a tunnel.)

And there's the East London Lines Shoreditch Station which is a tunnel on a viaduct, built to cater for future development on top of it.

 

[Tomnick]

It's so simple, i am amazed at this discussion. A tunnel is when one goes below earth that has not been disturbed. A cut and cover tunnel is therefore a bridge.

Not, in a number of cases at least, according to the Sectional Appendix!
--- old post above --- --- new post below ---

i think a bridge becomes a tunnel when it exceeds 20 metres in length

I haven't got exact figures, but I'm sure that an overbridge carrying a motorway (three lanes plus hard shoulder on each carriageway) would comfortably exceed 20 metres in width.

 

[asylumxl]

I swear this question was answered on here before and equated to something longer than a standard length of rail or something along those lines. 60ft was it?

 

[DaveNewcastle]

On the ECML, the tiny Askham Tunnel is definitely a bored tunnel, about 50mtrs long. Further north, there are the two urban roadbridges in Doncaster and Newcastle which are constructed spans of about 90mtrs, Balby Road and New Bridge Street, respectively, but both named in the Sectional Appendix as tunnels (or to be precise, 'Balby Bridge Tunnel' and 'Red Barns Tunnel'.

 

[snowball]

I haven't got exact figures, but I'm sure that an overbridge carrying a motorway (three lanes plus hard shoulder on each carriageway) would comfortably exceed 20 metres in width.

Such a bridge would be at least 32m.

 

[Cletus]

Is there a 'tunnel' between New Cross and St Johns in London that's 23 metres long?

 

[jnjkerbin]

Is there a 'tunnel' between New Cross and St Johns in London that's 23 metres long?

Unfortunately, the sectional appendix says where's only one tunnel (Tanners Hill Tunnel) and that's 87 yards or 80 metres!

 

[RPM]

Whilst Gerrards Cross tunnel was being built the line remained open. It wasn't so much a cut & cover structure, more of a cover & fill. Each day it became a couple of metres longer and the thought occurred to me that as there are specific rule book implications for tunnels, then we ought to be made aware of when exactly the structure would be long enough to count as a tunnel. However my enquiries came to nothing as nobody I asked was sure of the answer.

Then the damn thing collapsed so it became something of a moot point. :roll:

 

[Cletus]

Unfortunately, the sectional appendix says where's only one tunnel (Tanners Hill Tunnel) and that's 87 yards or 80 metres!

Thanks, my bad memory.
Just looked at google maps and there's only one tunnel there.

 

[Moonshot]

Ii it not that a bridge can be lifted out by a crane when it needs to be replaced? How do you do that with a tunnel ?

 

[snowball]

If a bridge is of in-situ concrete construction you can't lift it out with a crane, you have to break it up or blow it up.

Admittedly that's a type of construction not often used for bridges over railways.

The same would apply for a brick arch.

 

[Class172]

I haven't got exact figures, but I'm sure that an overbridge carrying a motorway (three lanes plus hard shoulder on each carriageway) would comfortably exceed 20 metres in width.

Such a bridge would be at least 32m.

Even more so where the crossing is at an oblique angle, such as here.