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  • HSR:Tradeoffs of having more or fewer axles, all else equal?

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

 #1455807  by tunatoro
 
Specifically with regards to high speed rail, what are the tradeoffs to having more or fewer axles for the same length and weight of train?

For example: do trains with fewer axles have less rolling resistance because they have fewer contact points, or do they have more rolling resistance because the contact points "dig in" to the rail more whereas having lower axle weight would allow the train to better "glide" across the rails without wasting energy deforming them? Do trains with fewer axles contribute to lower overall maintenance costs (including the track itself) because it is fewer moving parts to maintain, or do trains with more axles have lower maintenance costs due to lower track wear (due to lower axle load)?

I have noticed that different countries have different "standards" for how many axles their trains have, even though the trains may be the same length, have identical weights, travel at the same speeds, and even travel on the same tracks under the same rail regulations. For example, on one end of the extreme you have Talgo which makes trains based on jacobs bogies with only one axle each, so that a whole 200 meter train might only have 20-22 axles. On the other end of the spectrum, you have the Siemens trains like the Velaro which has 4 axles on each car for a total of 32 axles on a 200m train. Japanese shinkansens have axles like the Siemens trains. As an in between option, you have French trains from Alstom, which use jacobs bogies with 2 axles each, with 24 total axles on a 200 meter train.

To take a specific example, the Siemens Velaro and Alstom AGV trains both have 200 meter long variants, and they both weigh an almost identical amount just over 400 metric tonnes. The Velaro has 32 axles while the AGV has 24. It would seem to me that there would be significant engineering tradeoffs for the AGV to have 33% higher axle load. My understanding is that, like for trucks on roads, rail track wear and required track maintenance increases at an exponential rate relative to increased axle load. However, while I am pretty clear on the idea that higher axle loads will lead to more track maintenance, I don't know if that additional track maintenance is perhaps a minor expense compared to advantages of having fewer axles.

What are those tradeoffs between more and fewer axles for trains of the same length and weight?

As a bonus, it would also be interesting to learn about the tradeoffs that go in to deciding on how many powered axles to include. Whether you only power the locomotive axles, or whether you have distributed power where maybe half the axles are powered, or whether you power every single axle.
 #1456449  by tunatoro
 
Disappointed that I couldn't get any insight on this interesting technical question! Does anyone have advice on where someone might answer? For example, I first looked for a railroad Stack Exchange site, but found that it didn't exist. On the aviation Stack Exchange they are answering interesting technical question all the time.
 #1456502  by SemperFidelis
 
I would imagine that a route with more stations and grades would lean towards trainsets with more axles (powered axles) as the more wheels to "dig in", or with which to provide more traction (to try to sound a little more technical), the better the trainset's maintenance of speed whilst negotiating a grade and the better the trainset's acceleration whilst departing a station.

Ah, a thought occurs...and the plot thickens. The ability of the trainset to more quickly decelerate would be affected by the number of axles and would greatly affect an operation with many grades