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Post by markm on Feb 13, 2014 11:13:44 GMT -5
All,
With all the heavyweight experience that reads this site, I'm hoping someone can answer a question I've been researching unsuccessfully: Why did the railroads change from 3 axle to 2 axle trucks around 1950? Government regulation? Make them look like lightweights? maintenance issues with aging 3 axle trucks?
BTW: the "other company's" passenger trucks work nicely on the AZL heavyweight Baggage car and should work on the diner. The cars are not quite as closely coupled as the AZL trucks I'm still thinking about the heavyweight coach. The need for the stairs really limits truck swing.
Mark
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Post by Rob Albritton on Feb 13, 2014 21:47:31 GMT -5
2 axle trucks don't wear the rails as quickly as 3 axle trucks. That was a big advantage of the Lightweight cars - less weight to haul, and less track maintenance.
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Post by markm on Feb 14, 2014 12:12:27 GMT -5
Rob(A),
Thanks for the input. I would consider rail maintenance along with 1/3 fewer bearing to pack as a good justification. However, it's an answer that begs even more questions: Why order the cars with 3 axle trucks to begin with? Why wait decades to change them?
Perhaps there is no right answer. On the WP it's hard to find images of in-service passenger cars without 3 axle trucks, yet nearly all the surviving cars have 2 axle. On the SP, records indicate that the truck conversion was SOP during servicing. But images exist from the 1960s showing 3 axle cars in service on the Central Valley routes.
Mark
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Post by Rob Albritton on Feb 16, 2014 11:10:23 GMT -5
Hi Mark,
You need to think about the physics and geometry involved to understand the issues regarding the number of axles per truck.
Take a piece of paper and draw a curved piece of track. Now put a two axle truck on that curve, and then a three axle truck.
What you will find is that the wheels do not follow the curve on either design. The rectangle of the truck keeps the wheels at an angle that is not exactly perpendicular to the rail.
But the three axle design is worse than the two axle design because that middle axle puts even more stress on the truck and rails. Not only is it a longer truck (which makes the wheel / rail alignment worse than a shorter truck) but it also tries to keep the truck centered over the curve instead of following the curve.
I'm greatly over simplifying some complex geometry, but there is a reason that modern 3 axle diesels offer "steerable" trucks - it cuts down on rail and truck maintenance.
So why use a 3 axle truck in the first place? Something called axle loading. There is a reason the Heavyweights were called Heavyweights! They needed the extra axle to distribute the weight on a greater number of wheels, thus reducing the weight any one individual wheels was supporting. Some cars were later converted to two axle trucks (mainly lighter coaches and head end cars) mainly because they were running on tracks that had been upgrades to heavier rails that could support the higher axle loading. Why not convert them all? Well, it cost money to convert them. For some older cars it was more economical to let them end their lives with their original 3 axle trucks than spend lots of money to convert them. Again, there is no simple answer, but your example of older 3 axle coaches on branch lines makes sense: branch lines were the last to get upgraded heavier rails (in fact many branch lines were abandoned rather than spend money to upgrade them) and branch lines were a good place to let the old coaches live out the ends of their usefulness. So many railroads were created through mergers, its pretty easy to imagine that management knew decades in advance if a branch line would likely be abandoned one day, and planned expense accordingly.
BTW: Axle loading is also why there were "Heavy" and "Light" versions of locomotives - some railroads used heavier rail than others and could support heavier locomotives that produced more power, and haul more tons with the same man power.
enjoy
-Robert
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