Why do passenger cars have 6 wheels?

Model Railroader General Discussion (Model Railroader)
21

Sixteen had been experimented with very early:

Pullman2_1865 by Edmund, on Flickr

By 1876 Pullman simply states that twelve wheels had been ‘definitely adopted’:

Pullman2_1876 by Edmund, on Flickr

A major factor in the adoption of the all-steel car were the requirements of New York City, possibly the result of IRT subway impact that wood cars were not permitted in the North and East River tunnels and possibly including the Park Avenue ‘tunnel’ into GCT

Pullman_1907 by Edmund, on Flickr

Regards, Ed

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22

The answer to this question is a combination of most of the answers that have been given.

A typical all steel, mostly Pullman built, 80’ passenger car, in the 1910 to 1935 era, especially sleepers, diners, and lounge/observation cars, weigh about 90 tons, sometimes more. They typically have concrete floors anywhere from 2" to 4" thick.

That weight is twice the average loaded freight car weight from that era.

The weight, and the construction method, accomplished two highly desirable goals, safety and comfort.

The cars would no longer “telescope” into each other in crashes, derailed cars are more likely to stay up right do to the low center of gravity of the concrete floor and the heavy trucks.

For those of you who don’t know this, the trucks on railroad cars are not “attached” at the their pivot point, at least not in any way the will hold the trucks on in a derailment. A large pin and bearing plate on the car goes into a hole and bearing plate on the trucks.

But early on they saw the safety value of trying to keep the trucks on the car during derailments with passenger equipment. So safety chains are in place to do that.

This whole combination did prove much safer in derailments.

Comfort - The long wheelbase and extra axle of the six wheel trucks, along with carrying the high weight, created a much smoother ride, absorbed frog gaps, crossings and other tracks features with less lateral or horizontal movement transferred to the car.

It should be noted that a great many heavyweight RPO cars had 6 wheel trucks despite being much shorter. They had heavy interiors with all the mail sorting equipment, and a crew of postal workers had to ride standing up while sorting mail in the moving train - a smooth ride made that work easier and more efficient.

So, just think, a 20 car passenger train in that era was like a 40 or 50 car freight train in terms of weight and rolling resistance. Especially given the added factors of the longer wheelbase trucks having more resistance in curves, and factors of car length, resistance of spring loaded buffer plates and diaphragms, etc.

Modern lighter weight cars started to be built in the 1930’s. Stronger but lighter construction methods, better truck designs, brakes mounted on the trucks, all lead to many cars not requiring 6 wheel trucks and still providing a good ride. But many diners, dome cars, and some lounge cars of the streamlined era had 6 wheel trucks but in the newer truck designs.

Some railroads like the B&O were slow to adopt lightweight cars and actually modernized heavyweight cars to make them “look” streamlined.

Sheldon

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23

Safety concerns are surely a major factor. Like motor cars, I suspect that metal passenger cars also lasted longer and were easier to maintain than wood cars. Unfortunately, they were heavier and required more wheels to distribute the weight on the (crappy) rails of many RRs. But I’m only guessing :slightly_smiling_face:.

Simon

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24

Well, there have always been rail lines with “average” quality trackage. But in general, the pure physics of the metal has limited the weight each axle can carry thereby requiring more axles and/or larger diameter wheels for heavier cars.

Actually, back in the day, many of the big lines had exceptional track, and ran trains at pretty high speeds.

Sheldon

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25

:+1:

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26

Here in Canada, both CN and CP had poor trackage in the East in the 1950s. CN had many 8 driver locos to distribute the weight. And CP used a 4-4-0 on their mainline in the Maritimes up to 1960, in part due to the poor rails/bridges on some segments.

Simon

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27

It was really just the opposite here in the US during that time period. Roads like the C&O, B&O, N&W, and the PRR in the midwest and east had superior trackage for the most part. They were pulling long heavy freight trains with locos much larger and heavier than on Canadian roads, and passenger trains were long and heavy as well.

Traditionally there were two kinds of passenger trains in the US - the luxury “name” trains, and the “working” 2nd class trains. In the east especially, most of the name trains had smaller consists and carried little or no mail and express.

But the 2nd class trains would be long, heavy, and have as many baggage and RPO cars as they did passenger cars.

Out west some of the trackage was a little lighter built, but not by much. Sure, branch lines and secondary routes had lighter trackage, but the main lines needed to handle locos like UP Big Boy’s & Challenger’s, C&O Allegheny’s & T-1’s, B&O EM1’s & S1’s, N&W J’s, Y6b’s & Class A’s.

Big locos, doing BIG work, needed good trackage.

Sheldon

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28

1890s all-wood 80’ Pullman Palace cars all used six-wheel trucks. I believe part of it was the idea of more wheels providing a smoother ride, but also trucks at that time used some wood parts, so weren’t as strong as later all-steel trucks. Of course, the heavier all-steel cars needed the strongest trucks available, six-wheel all-steel.

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29

I enjoy the conversation… this stuff is fun to talk about

thanks guys

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30

We’ll have to have a discussion on Pullman cars with paper wheels sometime…

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31

The Allen paper wheels were discussed a number of times on the Kalmbach forums. These were interesting as an early example of ‘composite’ construction.

Wood is actually a useful structural material for absorbing shock, but it normally has ‘grain’ or imperfections in its structure. Paper is made from ground wood, and can be quite strong ‘in plane’, but of course is thin and buckles easily.

Allen’s idea was to make a kind of ‘plywood’ out of thousands of punched paper sheets assembled with strong glue and then clamped in a hydraulic press. The result was a strong, elastic disc, which was then assembled with a metal hub between two strong face plates and cross-rivets or bolts and given a metal tire with formed tread and flange. The process and result are in John White’s ‘The American Passenger Car’.

In the context of this thread… I don’t know of any four-wheel-trucked cars that ever got Allen wheels. That might be because any passenger service ‘important’ enough to be given them, royalty and all, might require the carrying capacity or number of seats in a car that would need twelve wheels…

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32

Yes, I learned about the paper wheels from John White’s book long ago. Apparently George Pullman used to like to confuse visitors when he would talk about Pullman cars now having ‘paper wheels’.

I believe most Pullman cars had six-wheel trucks in that era (1890s-1900s). I think by the time some railroads wanted long (80’) cars with four-wheel trucks, the paper wheels had long since stopped being made.

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33

I think the ability of cars to be made of lighter-weight materials but still meet buff and draft standards is what allowed an 85’ car to ride on 4-wheel trucks. By that time, I believe the next generation of resilient wheels (the ones for the PCCs without tread braking) were already starting to experience their own problems, and Eschiede was a further half-century in the future.

A problem with the Allen wheels was the tendency to brittle fracture. If any play developed laterally at the bolts, the wheel and rim would start to pound the edges of the holes, and (as with the critical parts of the Allied Full Cushion trucks) any cracks would be hidden by the metal plates until large sections of the wheel broke or crumbled, usually under load or lateral shock as in switches. It did not take long after this sort of thing started to occur that the Allen-wheel gold mine played out…

34

Questions:

Have I replaced my Cyc yet? 17 years later, no, I have not.

But regarding heavyweight cars…it is my impression that no one called them that until the “, lightweight” streamliners were invented. In comparison with the wood cars I just see them called “all steel cars” in the old literature…

35

Of course, patrons wanted the safety of all-steel cars over wood, so railroad advertising would tend to emphasize that aspect. From a railroader point of view though, the fact that they were so much heavier did create a need for much larger engines. 4-4-2s were replaced by 4-6-2s and later 4-8-2s. So the weight was a big factor to the railroads. Would be interesting to know when “heavyweight” was first used in print; but since railfan books weren’t really a thing until the 1930s (when streamliners were introduced), you’d almost have to look into actual railroad company archives to see.

36

All Heavyweight cars have 6 axle trucks. That’s why they’re called “Heavyweights”.

Some streamlined Passenger cars from Union Pacific, Monon, and Gulf Mobile & Ohio also had them.

37

I think ‘heavyweight’ is more a railfan term, to distinguish the regular twelve-wheel clerestory-roof steel cars from the lightweight stuff.

I found a reference from Fred Westing last night (in ‘Apex of the Atlantics’) that car weight per passenger actually decreased substantially going from wooden construction to ‘all-steel’ P70s. This savings may have applied to open-section sleepers, although the accommodations per car would have been roughly the same; I suspect there would be even lower ‘savings’ in cars carrying fewer passengers in larger rooms.

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38

In a way, yes it is.

Streamlined cars were lighter than Heavyweights. But Heavyweights rode like a “Cadillac”.

39

Part of that might have been the ballasted deck – heavyweight often had concrete floors that as I recall could be up to 4" thick. That lowered the c.g. and roll center of the car, insulated some sound and vibration, and gave inertial mass to simplify accommodation of the secondary suspension.

The best ride I ever had in the early-'70s NEC was on one of the P70s – I have no idea whether it had the ‘improved’ modern trucks or not, but it had the purple velvet and bronze seat framing, with dim light like a mausoleum. With 2" of concrete in the deck you could (vaguely) hear the familiar trips, scurries, and flange noises going over crossovers at speed, but that did not translate into any kind of vertical jounce or sideways bobble. That’s the only short-run railroad coach I ever fell asleep in! :sleeping_face:

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I remember riding in a Heavyweight coach at a railroad museum, the ride was smooth and very few if any bumps were felt. I think the cars was either ex Santa Fe or Southern Pacific…

I also recall hearing Heavyweights had very comfortable seating…