Axle loading?

Ask a real train driver or fireman questions about how engines work, railways operate or anything else. If we do not know the answer, we will find out!
Post Reply
Warren Beck
Posts: 31
Joined: 03 Sep 2011, 19:36

Axle loading?

Post by Warren Beck »

How is the maximum axle load determined for a line? I stay by the Caledon/Overberg branch, the max axle load is 18.5 to Caledon and 16.5 to Bredasdorp which seems very low. So only a class 33, 35 or 36 to Caledon and a 33 or 35 to Bredasdorp? What is it about the infrastructure that limits the weight it can carry?
User avatar
John Ashworth
Site Admin
Posts: 23606
Joined: 24 Jan 2007, 14:38
Location: Nairobi, Kenya
Contact:

Re: Axle loading?

Post by John Ashworth »

I'm sure there are experts who will give a complete answer. I would guess that the weight of the rail is one factor, and the load-bearing capacity of bridges and other infrastructure another.
Image
User avatar
Steve Appleton
Site Admin
Posts: 3605
Joined: 23 Jan 2007, 14:14
Location: Johannesburg, South Africa

Re: Axle loading?

Post by Steve Appleton »

John is correct.
The rails, sleepers and track-bed are the biggest cost factors to consider when building a line. It is about a construction cost vs performance-income ratio. There is no point in building a hugely expensive line when the traffic volume and revenue is low, as is the case with most branch lines.

Reducing construction cost means using lighter and cheaper rail. It means using cheaper sleepers laid further apart and it means using less hard-core and compaction in the track-bed or base formation. In addition lighter and cheaper structures are used wherever culverts and bridges are needed.

It is also about how good the alignment needs to be maintained. Slow-running trains can tolerate misaligned track better than high-speed ones. Heavier track with deeper ballast keeps its alignment better between maintenance visits than light track. Some misalignment on lightly-used slow-speed branches laid with lighter track is tolerable and this factor contributes to reducing construction costs.

So, in a nutshell. Heavier rail (measured in kgs/meter) allows heavier axle loads without permanent deformation or cracking due to fatigue as the rail bends and springs back under load. Harder and more expensive steel in the rail (and a heavier rail-head profile) reduces squashing, peening and deformation of the rail-head under load. Closer sleeper (tie) spacing (more sleepers/km) and heavier sleepers will carry more load down to the track-bed and support the rails better. Concrete sleepers are better than steel and wood and last longer. Lastly the strength of the track-bed when constructed and its drainage is an important factor to ensure the bed can carry the intended load without permanent deformation and subsequent track misalignment. Branch lines generally use less expensive and lighter load-bearing earthworks compared with main-lines.

Engineers determine the maximum permitted axle-load based on a combination of these factors and a wealth of experience with the causes of track failures. Though, in some cases higher axle loads may be permitted on some track compared with an amost identical track elsewhere when higher maintenance costs and more frequent maintenance visits can be justified by the revenue and traffic volumes. In other words, it is not just about the construction strength and cost but also about the on-going maintenance costs. Lower axle-loads may be specified just to reduce wear and tear and thus reduce maintenance on little used lines.
"To train or not to train, that is the question"
Warren Beck
Posts: 31
Joined: 03 Sep 2011, 19:36

Re: Axle loading?

Post by Warren Beck »

A gentleman and a scholar. Couldn't ask for a better reply.
User avatar
Philip Martin
Posts: 102
Joined: 24 Aug 2014, 17:51
Location: Pennsylvania, USA

Re: Axle loading?

Post by Philip Martin »

The Kursk-Kharkov railway line in the Ukraine in the 1880s limited the number of axels in a train, apparently for weight reasons, although having more axels spreads out the weight. Below is an excerpt from the wiki article on the 1888 wreck of the Russian imperial train near Borki.

"The imperial family was en route from Crimea to Saint Petersburg. Contrary to railway rules of the period that limited commercial passenger trains to 42 axles, the imperial train of fifteen carriages actually had 64 axles, well above the safety limit. Its weight was within the limits set for freight trains, but the train actually travelled at express speeds. It was hauled by two steam engines, a combination that caused dangerous vibrations that, according to Sergei Witte, directly caused the derailment."
Attachments
url.jpg
url.jpg (124.28 KiB) Viewed 2942 times
User avatar
John Ashworth
Site Admin
Posts: 23606
Joined: 24 Jan 2007, 14:38
Location: Nairobi, Kenya
Contact:

Re: Axle loading?

Post by John Ashworth »

Thanks, Philip. Not sure that this accident was precisely about axle weight but more about the total weight of the train, as well as the length and speed of the train. As you say, the number of axles does not make the axle loading any greater. In South Africa the length of a train is often spoken of in terms of axles, so a 42-axle train would be 7 cars while a 64-axle train would be 16 cars.

On a completely different tack, during the 1984 famine in Darfur, Sudan, the UN and aid agencies wanted to ship food to Darfur by rail. However it was found that the track west of Babanusa was too light for the more powerful diesel engines which had a high axle weight, so the trains had to be split into smaller trains hauled by less powerful locos of lower axle weight. Somebody thinking outside the box decided to rehabilitate six steam locomotives, which were as powerful as the big diesels but had lower axle weight. I believe Phil Girdlestone led the rehabilitation project, but I don't know how long the steam locos continued to work.
Image
User avatar
John Ashworth
Site Admin
Posts: 23606
Joined: 24 Jan 2007, 14:38
Location: Nairobi, Kenya
Contact:

Re: Axle loading?

Post by John Ashworth »

Sorry, let me clarify my own post and explain how I calculate that 42 axles is 7 cars, when of course a normal car has only 4 axles. Since a passenger car is generally significantly longer than the average freight car, South African railways allocate a nominal 6 axles to passenger vehicles when calculating the train length, so a 42-axle "equivalent axle length" passenger train has 7 cars. 4m is assigned to each axle, so a train of 42 equivalent axles would be 168m long. For calculating the axle load of the train the actual number of axles is used, and a 7 car train actually has only 28 axles, if that makes sense.

A normal freight train could not be 42 axles long, as that would be 10.5 cars, unless it contained, for example, 9 normal cars (36 axles) plus one passenger-type vehicle or one of certain specialised freight vehicles which are also assigned 6 axles for calculating train length.

I hope I got that right! Steve?
Image
User avatar
Philip Martin
Posts: 102
Joined: 24 Aug 2014, 17:51
Location: Pennsylvania, USA

Re: Axle loading?

Post by Philip Martin »

Thank you for the interesting information, John. I wonder how total weight could cause the derailment, since it is spread out. This derailment apparently occurred on a fill, not a bridge. But I suppose the officials knew what they were doing when they put in their axel limit. Reading about the Borki wreck I get the feeling that the people who investigated it weren't competent to find the actual cause.
Food to Darfur- steam engines to the rescue. Good story, John.
User avatar
Steve Appleton
Site Admin
Posts: 3605
Joined: 23 Jan 2007, 14:14
Location: Johannesburg, South Africa

Re: Axle loading?

Post by Steve Appleton »

Regarding "equivalent axle length", John is correct. Train controllers need to know the length of a train to determine whether that train will fully fit into a crossing loop or siding to allow another train to pass or cross it. One way of doing this is to compare the "axle length" of the train with the "axle-length" of the intended loop. So, a freight train of 8 bogie wagons would be "32 axles long" and the loop must be somewhat longer than this for it to fit.

The actual axle count does not work for passenger vehicles because they are about 50% longer than a freight wagon. So, the train length in axles is fudged by clalculating passenger vehicle axle-length as though each possesed 6 axles. Thus an 8 coach passenger train length, although still 32 actual axles long, would be calculated as being equivalent to a freight train of "48 axles long". This ensures that the longer train length is taken into account when determining whether to use a particular loop for a crossing.

All of this is very rough and ready because, in reality, railway wagons and coaches do vary in length by type. This method also takes no apparent account of the quantity, length or type of the motive power that is hauling the train. Such errors could become quite large for longer trains and I have to wonder whether this measurement method is practically useful these days. It would be far easier, because the TFR IT system knows the train consist and its wagon types, to use the system to compute and accurately report each train's length in metres.
"To train or not to train, that is the question"
User avatar
Philip Martin
Posts: 102
Joined: 24 Aug 2014, 17:51
Location: Pennsylvania, USA

Re: Axle loading?

Post by Philip Martin »

I had experience with a short line railroad in the US, the Lehigh & Hudson River, that had a maximum of 135 cars on its freight trains. Being in towers, I didn't know about other factors
Post Reply

Return to “Railway Operations - ask a question about how railways work..”