Railway General Knowledge.

[DerekC]

Two small tubes would be stronger than one large one?

 

[krus_aragon]

Two small tubes would be stronger than one large one?

It could stand to reason (and that had been my assumption too), but no.

 

[341o2]

would this be due to a similar requirement made to Brunel when bridging the Tamar in that the Admiralty stipulated that the bridge must be at a certain height (in the case of the Royal Albert Bridge 100ft) above river level to permit the passage of a fully rigged sailing ship. As well as the height, there was the distance between the piers, hence the need for a rigid structure

 

[krus_aragon]

would this be due to a similar requirement made to Brunel when bridging the Tamar in that the Admiralty stipulated that the bridge must be at a certain height (in the case of the Royal Albert Bridge 100ft) above river level to permit the passage of a fully rigged sailing ship. As well as the height, there was the distance between the piers, hence the need for a rigid structure

You are correct that the Admiralty insisted that the bridge be high enough that it would not impede the passage of ships (like the Menai suspension bridge). But I suspect you're describing the fact that there are separate tubes either side of the central tower on Britannia rock, which isn't quite what the question was asking. Your mention of rigidity, however, is tangentially connected to the answer.

Stephenson had already decided that a tubular structure was the solution. I'm asking why he said that two sets of tubes (one for each running line) were needed.

In case any readers aren't familiar with the structure of the (pre-rebuild) bridge, here's an engraving showing the entrance to the two tubes:

 

[DerekC]

From memory the tubes were prefabricated close to the site, floated out on pontoons, located into slots in the piers and then jacked up into position. Might one big tube have been to large and heavy to allow this construction method?

PS - I wonder why the train is running wrong line!

 

[krus_aragon]

From memory the tubes were prefabricated close to the site, floated out on pontoons, located into slots in the piers and then jacked up into position. Might one big tube have been to large and heavy to allow this construction method?

Your memory is correct about the method of assembling and erecting the tubes, but Stephenson didn't say that was the reason for having separate tubes for thte up and down lines.

PS - I wonder why the train is running wrong line!

Yes, me too. There's another engraving with wrong-line running, depicting the other end of the bridge. And there's an engraving which appears to be a mirror image of the first, but while it puts the train of the correct track, the curve of the railway is now wrong. But that's not directly relevant to the current question.

A hint to the answer: try to imagine what would happen to the bridge if it only had one tube for both lines...

 

[DerekC]

Did Stephenson say it would sag too much? I would have thought that could be overcome by putting a longitudinal diaphragm into the tube between the two tracks, but perhaps that would have made it harder to fabricate.

 

[krus_aragon]

Did Stephenson say it would sag too much? I would have thought that could be overcome by putting a longitudinal diaphragm into the tube between the two tracks, but perhaps that would have made it harder to fabricate.

You're dancing ever closer to the answer, but the concern wasn't about sagging, per se.

 

[Trackman]

some sort of resonance damping?

 

[krus_aragon]

some sort of resonance damping?

No, sorry.

The issue is to do with the consequences of putting two running lines in a single tube.

 

[Trackman]

No, sorry.

The issue is to do with the consequences of putting two running lines in a single tube.

Something to do with air turbulence?

 

[krus_aragon]

Something to do with air turbulence?

Another good thought, but no.

Rather than problems with two trains being in the tunnel at the same time, this problem would manifest itself when one train ran through the single tube.

 

[Trackman]

Another good thought, but no.

Rather than problems with two trains being in the tunnel at the same time, this problem would manifest itself when one train ran through the single tube.

Uneven weight distribution? That would cause problems!

 

[krus_aragon]

Uneven weight distribution? That would cause problems!

Yes! The issue was with an off-centre load twisting the tubes.

Going from memory (as I don't have my copy of Baughan's History of the Chester & Holyhead Railway to hand) Stephenson was concerned that if two tracks were placed in the same tube, the weight of a passing train would apply a torsion force that would try to twist the tube. By having one tube for each track, he could ensure that the track was aligned centrally, and avoid any significant lateral forces arising from the train passing through.

(I'll edit the above response with a more direct quote this evening.)

Your floor.

 

[Trackman]

Yes! The issue was with an off-centre load twisting the tubes.

Going from memory (as I don't have my copy of Baughan's History of the Chester & Holyhead Railway to hand) Stephenson was concerned that if two tracks were placed in the same tube, the weight of a passing train would apply a torsion force that would try to twist the tube. By having one tube for each track, he could ensure that the track was aligned centrally, and avoid any significant lateral forces arising from the train passing through.

(I'll edit the above response with a more direct quote this evening.)

Your floor.

Got there in the end! It was the single train in the tube that made it click.
Cannot think of a question off the top of my head so open floor

 

[Calthrop]

I'll set one, if I may...

I'm very fond of the narrow gauge: so...

Which narrow-gauge line in Great Britain, with a public passenger service; did not name its locomotives?

 

[martinsh]

Corris Railway

 

[Calthrop]

@martinsh -- you're on my case re things narrow-gauge and preoccupation with, good and proper ! The Corris, it indeed was. Your floor.

 

[martinsh]

Open floor

 

[341o2]

A British public narrow gauge line for many years uniquely escaped a piece of legislation which applies to all passenger railways. To make it a bit more difficult
1. Which railway
2. What was involved
3. How did this occur
4. What was the railway obliged to do until it was decreed that this situation must come to an end

 

[Calthrop]

1) Ffestiniog Railway

2,3) The 1832 Act of Parliament allowing inauguration of the railway, did not (unlike Acts of Parliament for later railways) make provision for closure or abandonment of the FR; after traffic ceased on the FR in August 1946, the FR Company could thus not officially abandon the line.

4) Track and infrastructure had to be left in place: the utterly broke FR Co. did not have the money which would have been necessary to have an amending and repealing Act of Parliament passed, to permit them to abandon officially. This is seen as having saved the line "on the ground", until those seeking to preserve the FR were in a position to clear the legal and financial tangle and take over the railway.

Open floor if correct, please.

 

[341o2]

Not the answer I was looking for, I'm not sure that the Ffestiniog is unique in no trains running but not abandoned, and to make no4 clear, this refers to the operation of trains.

I believe something similar occurred briefly on other lines such as Leighton Buzzard,

 

[krus_aragon]

I had thought about the Talyllyn's issue with limited clearance for (accidentally) open doors in tunnels, which was resolved by slewing the track over and locking all the doors on one side of the carriages. But on checking, that was found and resolved when the line was inspected prior to opening.

 

[341o2]

You are right in that the railway concerned is the Talyllyn

 

[Calthrop]

I understand that at the Talyllyn's inception in 1866, the appointed Board of Trade inspector was at first -- for various reasons -- reluctant to give permission for opening for public traffic; but a couple of months later, after the railway had addressed some of the problems, he cautiously and provisionally gave the go-ahead. This assent left aside some things which should have been covered -- the railway was lucky in that for many decades, no serious accident occurred which would have revealed the omissions, with who-knows-what consequences. It's not clear to me in any detail, what the question's particular "piece of legislation" actually was -- but presumably, the inspector's "provisional OK" should have dealt with the matter (but didn't).

 

[341o2]

The relevant legislation did not exist when the Talyllyn opened.
Also no4 probably relates to the post preservation era

 

[krus_aragon]

Was there a lack of a continuous brake along the carriages?

 

[341o2]

Yes, you have answered no2. From the 1889 Act, an important part was that all passenger trains must have a continuous brake, until the end of the 20th centuary, the Talyllyn was exempt.

 

[341o2]

To complete the post
3- When the railway was made aware of the 1889 Act, McConnell pleaded exemption on the grounds of low speed and light traffic. Also, he claimed that the railway could not afford to implement the requirements. This was granted, presumably that the Talyllyn sunk into obscurity helped, had it prospered or drawn attention to its existence the Act could have been enforced.

4- In the absence of a continuous brake, all passenger trains must have a manned braked vehicle at the lower (Tywyn) end of the train. The railway also installed an electric circuit, all vehicles had wires and a plug at each end, a breakaway would cause a break in circuit and sound an alarm to warn the loco crew and guard. I suggested while volunteering that if the guard was expected to control a runaway by handbrake, then it would be good idea to practice with a real train, so this was arranged with no5, some wagons and a van near Brynglas one evening.

 

[341o2]

open floor