Why do passenger cars have 6 wheels per track? Are they heaver than a boxcar?
Sleeper
Passengers are more delicate and prone to complaining about rough rides than freight.
For the last 60 years or so, passenger cars have been built with four-wheel trucks. They are known as âlightweightâ cars. In the earlier part of the 20th century, âheavyweightâ all-steell cars were built. They normally had six-wheel trucks to reduce the per-axle load. Some of the smaller heavyweight cars (say 60-feet long) were converted from four to six-wheel trucks to compensate for the weight involved with the later addition of air conditioning.
Mark
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When I eventually get my 12" to the foot passenger car, if it doesnât already have 6 wheel trucks, I am going to re-equip just for that purpose.
I recently saw a Trains and Locomotives episode that showed a bunch of what looked like older, long heavy weight cars with 4 wheel trucks. 6 wheel Pullman type cars, but with 4 wheel trucks. Did the lines ever redo these cars and change them to 4 wheel trucks? Anyone ever do this to their models?
Passanger cars have six wheels so they can roll on the track.
Well somebody had to say it
Mark has expressed my own notion of this. The term âheavyweightâ must have had some meaning in the context of passenger cars at the onset of WWII and earlier. It was that they were truly heavy cars, and when loaded, even heavier. The three axled trucks made for a safer ride in terms of accidents caused by derailments and other problems associated with overloaded rails below them. This was afforded by the weight being distributed over two additional axles per car.
For example, the SP lightweight (4-axle) baggage/coach class 77-CB-1 weighed 114,000 lbs., while the shorter, non-airconditioned heavyweight (6-axle) baggage/coach class 72-CB-2 originally weighed 136,500 lbs.
Mark
A lot of it has to do with weight. Heavyweight cars were called so becauseâŚwell they were heavy. They traditionally rode on the 6 wheeled trucks because the weight was better distributed over 6 wheels versus 4.
An oddity in the âmodernâ era of lightweight passenger cars is the Santa Fe Hi-Level diner. Due to the immense weight, this car rode on 6 wheel trucks instead of the normal 4 wheel trucks the other Hi-Levels rode on. Interestingly all Superliners ride on 4 wheel trucks.
The older, âheavyweightâ cars were an attempt at making the ride smoother for passengers by increasing the mass of the cars making them less bouncy over the occaisional piece of rotten track.
In fact, in addition to the steel construction these cars had concrete poor into their floor pans to increase the weight.
The 3-axle trucks were needed to distribute the weight over more rail. Beacuse of this it would be very unlikely that a heavyweight car would have its 3-axle trucks replaced with 2-axle trucks (not to say that this never happened though).
Eventually they figured out that tuning spring rates to body mass could result in a good ride but without the need for all that extra weight. Since the extra weight put more wear and tear on the track and required more fuel to hoist it up a grade, and also wore down the brakes more when slowing down (plus was slower to accelerate to speed) light weight cars became popular amoung the railroads whose marketing departments portrayed them as more âmodernâ to the railroad traveling public so the heavyweights, when not scrapped got relegated to secondary (or tertiary) service.
At least thatâs my story (and Iâm stickinâ to it - for the moment anyway).
Regards,
Charlie Comstock
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As far as the Santa Fe goes, they had some stainless cars (some would call lightweight) with 3 axle trucks. The first that comes to mind was the hi level diner, which was the largest and heaviest dining car built (I am pretty sure it was the heaviest). Then there were the Budd âBig Domesâ, which is quite a large car itself. The last ones that come to mind were the Budd built business cars, such as the âTopekaâ. They were quite heavy due to all the equipment that made them a self sustaining car (generators and what not).
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Hi,
The older cars are often termed âheavyweightsâ today, as that is exactly what they were, in comparison to the Modern cars of the 50s and forward. Being heavier, they needed that extra axle on each end to spread the weight, and to give a smoother ride.
ENJOY,
Mobilman44
BCSJ:
Youâre right about weight distribution and unsprung weight ratio, but I doubt the heavyweight cars were ballasted simple to make them heavy. I donât recall the concrete floors being that thick, and I think they were more âhow to make a quiet, durable floorâ than âhow to make a car heavierâ. Iâm pretty sure streamliners had concrete floors, too. (could be wrong, lost my Cyc a while back) I think the extra weight was just an artifact of having all the strength in the underframe, as these cars did, and was something not considered a problem (since it improved the ride) rather than a thing actively sought.
There were some lighter cars made, where the sides did some work. The Erieâs Stillwell cars were this type, and had four wheel trucks.
The principal reason for the development of âheavyweightâ cars was to replace wooden cars with all-metal cars to provide more protection of passengers from the danger of collision, fire, and derailment. For example, there were incidents where one car would âtelescopeâ into another car, sort of like a syringe. It took about 40 years to develop and accept the use of lighter metals and improved design for what became âlightweightâ metal cars.
Mark
Because wheels work better on steel rail than skids.
RETAININGLY yours
R. T. POTEET
Let me turn this topic upside downâŚ
It has always been my understanding that the term âheavy weightsâ referred to passenger cars made with steel vs. the old wooden cars.
The purpose was safety. Train wrecks often resulted in the old wooden cars âtelescopingâ into each other, leaving nightmarish results. People were simply mashed inside the cars. This was so horrific that railroads went to steel to up the level of safety. It worked and passenger traffic picked up.
As a structural engineer 6 wheels definitely would spread the load which I agree is a good idea, and likely necessary due to simple wear and tear. But here again, being contrary to common opinion, the resulting ride from 6 wheels was preferred by the ridership.
I once listened to some knowledgeable railroad guys who were debating this topic and the consensus was that 6-wheel trucks âsmoothed outâ the frogs at switches and gave a smoother ride.
So, it seems to me to be valid that 6 wheeled trucks provide a smoother ride, good tracking through switch frogs, as well as disperse wheel loads.
Youâll note that six-wheel trucks came to be used early on Pullmans, even before the heyday of Allen âpaperâ wheels, and were then common on steel cars well into the 1950s modernized with roller bearings and rubber bushings for high-speed operation. They were used where spreading the weight over six axles gave lower axle load.
They would have to be carefully designed with lateral on the middle axle to be âsmootherâ through frogs; the advantage with lateral compliance probably involved the four rather than two bolster ends for secondary suspension (which in many Pullmans involved a reversed semielliptic leaf spring to the outside sills for greatest lateral side bearing).
See Whiteâs The American Passenger Car (volume 2 for those of you with multiple-volume editions) for a discussion of trucks, culminating in 1950s designs of outside-spring-hanger (OSH) trucks.
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Six wheel trucks also would have been appropriate with the lighter rail of those days. Also cement floors would have lowered the center of gravity in those cars.
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If I remember correctly, at least some of the PRR P-70s had 2" of concrete in their decks, and rode on 4-wheel trucks. I rode one in the early '70s that still had the bronze fittings and purple velvet upholstery, and the thing ran perfectly smooth and as quiet as a mortuary while other cars in the train were not.
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The change from all-wood construction with truss rods to all-steel was in large part due to the US Post Office. I donât remember the exact dates, but I believe in the 1890s the Post Office decided that any Railway Post Office car had to have a steel underframe for safety, followed in the early 1900s by requiring them to be all steel. Since the postal contracts were a big source of revenue, the railroads did what was required and went with steel RPOs.
But then, how do you explain to your riders that the wood cars were safe enough for them, when the RPO clerks were riding in all-steel cars? Pretty soon, it was just better publicity to start buying all-steel passenger cars. These cars were much heavier than their wood counterparts had been, which lead to the development of larger steam engines.