March 16, 2004

Trains can't stop - or can they?

Patrick Crozier | Fragmentation

In a comment to an In Brief item I found myself (in response to the whole fragmentation debate) asking the question: what is it about replacing tarmac with steel rails and rubber tyres with steel tyres that means that whoever operates the vehicles must also control the infrastructure?

And me being me I immediately set about trying to answer it.

My understanding is that one of the big problems with trains is that they find it difficult to stop, (remember "The Railway Children" anyone?) Specifically, they can't stop in the space that the driver can see. Therefore, they need signals to tell them that the stretch up ahead is clear.

But what about trams? They have to operate in mixed traffic conditions ie with normal road vehicles. They don't need signals. They seem to stop all right. So, why don't they have this problem? Is it the speeds involved? Or is it because they are lighter? And if it is because they are lighter why not make trains lighter and get rid of the signals?

Trackbacks

Train Brakes
Brian Crozier over at Transport Blog muses about train stopping ability a subject that I didn't know much about but this referenced article makes all clear (it pays to read comments). One improvement that presents itself immediately to me is...
Flit(tm) on March 16, 2004

Train Brakes
TMLutas and Brian Crozier write about train brakes. Here's an idea: add an "anchor" (or multiple ones) to the bottom of all train cars: in a real emergency, release them to decrease stopping distance: Notes: In a real emergency,...
Ranting and Roaring on March 16, 2004

Train Brakes
TMLutas and Brian Crozier write about train brakes. Here's an idea: add an "anchor" (or multiple ones) to the bottom of all train cars: in a real emergency, release them to decrease stopping distance: Notes: In a real emergency,...
Ranting and Roaring on March 16, 2004

Comments

It's not clear to me that the long stopping distance of a train is sufficient to explain why vertical integration should be better. It has implications for the sort of rules that the track owner imposes on the train operators. But as far as organisation is concerned, why should that be fundamentally different from requiring pilots to obey the instructions of air traffic control?

Posted by Andy Wood on March 16, 2004

Your physics is a bit wonky there Patrick. Weight (mass) doesn't affect stopping distance provided the brakes are sufficiently powerful. The limit on deceleration is the coefficient of friction at the wheel/rail interface, which is similar for trams and heavy rail. Heavier = more friction hence greater stoping force, which compensates for the additional inertia.

Trams can stop within sighting distance because:

1) The speeds involved are controlled to be relatively low;
2) Trams often have some form of non-friction-based braking system as a backup, such as track magnets.

Posted by Steve Brown on March 16, 2004

could you repeat that andy? I didn't quite catch what you said the fifth time.

Posted by Neil on March 16, 2004

Neil, I think you meant to address that to Steve.

In reply to Steve, Patrick's physics was right the first time, as this link will tell you. Trains don't skid to a stop. Relevant quote:

A second counter-intuitive situation exists with train brakes. Reconstructionists are accustomed to ignoring vehicle weight when calculating stopping distances. This assumption has validity for vehicles that skid to a stop. Trains on the other hand are designed with a maximum brake force that is below the force necessary to lock the wheels of an unloaded train. The effect of this is that maximum braking force is the same for loaded and unloaded trains and stopping distance is roughly proportional to weight. Stated another way, a train weighing twice as much will take about twice as far to stop. While this idea may be counter intuitive, it of course makes perfect sense for a vehicle with fixed maximum braking force.
Posted by Andy Wood on March 16, 2004

For those of us who are lousy at physics (I managed to fail dismally at O level) trains are heavier and faster - both will have an effect on braking distances (inertia and velocity). The signalling system is designed around the longest braking distance of the trains using the route at maximum linespeed.

As for organisation - there is no real parallel elsewhere. The one with air traffic control only applies to the driver/signaller relationship. With railways, there is the driver/signaller/engineer interface. Also, the sky doesn't break down as much as track and signalling ;-)

Posted by Mark Ellott on March 16, 2004

But why should the introduction of engineers be such a fundamental difference that it should require vertical integration? Why can't the engineering requirements be accommodated in the contract between track owner and train operator?

The only problem that comes to my mind is that the track may have to be closed for maintenance or an emergency, thereby disrupting the operator's schedule. But that happens with air traffic control too. The grounding of thousands of planes on 9/11, for instance, seemed to happen quite smoothly without vertical integration.

Posted by Andy Wood on March 16, 2004

This one is being picked up in parallel on another thread - I guess the best way of putting it is that with air traffic there is only a partial parallel with the railways. The airways are three dimensional. Of course, when signallers claim parity with air traffic controllers, Network Rail managers point this out to them (it's a two way argument). Air traffic manages aircraft movements - it does not have to juggle maintenance and renewals requirements into a complex "rules of the route" regime - keeping trains running and the infrastructure maintained.

On the other discussion there was this question of why there is no theoretical model - I'm not entirely sure. What I do know, though, from personal experience, is that it works more smoothly if the people who run the trains also manage and maintain the tracks and signalling - sorry I can't be more scientific. Sometimes, I think, there isn't any science - it just "is".

Posted by Mark Ellott on March 16, 2004

If the erstwhile Railtrack was anything to go by, the purpose of vertical integration is that the people who maintain the track have the same goals as those who run the trains. They both want the organisation - their own, the same organisation - to succeed. But as things stood, Railtrack had relatively little to lose if Virgin's desire for West Coast Main Line modernisation went up the spout.
Under British Rail, the track maintainance part of the operation cared about the running of the trains, but neither they nor those responsible for the trains had much to lose from inconveniencing passengers.

Posted by James on March 16, 2004

Apologies for the 5 posts, I have no idea how that happened.

Andy, thanks for the pointer to the interesting article. The statement "Trains on the other hand are designed with a maximum brake force that is below the force necessary to lock the wheels of an unloaded train" may be true for mile-long US freight trains, but it definitely is not true for disc-braked UK passenger multiple units which are quite capable of locking up all axles even under good adhesion conditions. For passenger trains and trams, the limit on deceleration is still set by the railhead friction.

Signalling design for freight trains and mixed traffic is indeed a greater challenge and there are few rail networks with such a wide selection of traffic running on shared infrastructure as in the UK.

Posted by Steve Brown on March 17, 2004

I'm not a train enthusiast like some here but I do enjoy big machinery powering along. Any programme about, say, the industrial revolution with pumps, engines, spinning jennies or moving kettles ensares me.

I was fortunate to stumble upon a programme on the Discovery Channel last night called Speed Machines. This episode was about the great speed battles between the LMS and LNER in the 30s.

One terrific section detailed the LMS's Coronation Scot going for the 100 and 150 mile records plus the absolute top speed record. They figured out that London to Crewe was the best on their track for this feat. Further north would have meant dragging up the long climbs through the Lake District and over the Borders.

The fastest section was actually in the run into Crewe itself. However not only were they supposed to stop but there was a 25mph speed limit over the points prior to the station to content with.

The train hit 112.5mph before they slammed on the brakes with just 2.5 miles left to go to Crewe station.

According to this website the train did the last 1.1 miles to the stop at Crewe in 1 minute and 19 seconds. That's not a very long time to bring a huge great train to a dead stop.

Yikes!

Posted by Mark Holland on March 17, 2004

It must've been fun for any people on board drinking coffee...

Posted by randolph on March 17, 2004

The LMS run is legendary - apparently the train took the 25mph restricted double slip at the throat of crew at 75mph, and I believe that more than soup got spilled in the dining car......of course it was a PR triumph and in the days before Health and Safety were quite the issues they are today, no-one seemed too bothered that they were lucky not to end up with a pile of mangled steaming wreckage outside Crewe station.

Posted by andy wakeford on March 17, 2004
Signalling design for freight trains and mixed traffic is indeed a greater challenge and there are few rail networks with such a wide selection of traffic running on shared infrastructure as in the UK.
Indeed. And some of the networks that match the UK for mixture of traffic have some serious momentum to master. I commend a visit to the Burlington commuter line from Chicago to Aurora during the afternoon rush hour, which is sometimes spiced up by a coal train off the Powder River Basin showing up. Posted by Stephen Karlson on March 23, 2004

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