Couple of questions about the article “How Heavy Can Freight Cars Get” (March 2006). The maximum car weights of 263k, 286k, and 315k lbs. are freely tossed about without mentioning how they were chosen. Is there anything sacred about them? Why were they not, for example, 265k, or 320k lbs? Would three axle trucks make a difference or are they too expensive?
This has been bounced around a good few times before… the chosen weights are somewhat arbitrary, but are related to maximum axle loadings and also to infrastructure design capacities (rail, roadbed, bridges, etc.).
Six axle trucks are a pain in the tush.
If one were to consider a 320k lb maximum freight car weight, equivilant 160 tons; with the 85 ton heavy weight passenger car utilizing 6 wheel trucks. The six wheel trucks made for the smoother ride. In the case of increasing freight car weight limits and using six wheel trucks, the rails, ties and roadbed, are taking a beating now from four wheels, that in some cases is extreme for some trackage. My personal opinion is that the trackage (rail, ties and base) has reached it capacity until a more substantial combination is implemented.
At this time with prior clearance from a receiving railroad you can have 315,000 pounds on four axles. Bridge loading, wheel sizes, bearing sizes, and rail contact stresses are the major factors.
Six wheel trucks would be a big complication, in truck frame assembly, brake rigging, etc. The three piece truck with two axles will be the standard as long as any of us are alive.
I had been wondering the same thing as Eastside. After reading the other posts, I took a look at my copy of John H White’s book “The American Railroad Passenger Car” Vol 2. He reports six axle trucks were developed to support the increasing weight of ‘palace cars’ like Pullman’s, but it had as much to do with reducing damage to the axle bearings and wheels as it did with reducing damage to the track. It was also thought that extra brake shoe area would increase the service life of the shoes and improve stopping distance.
There was also a theory about the long wheelbase of a six-axle truck improving the ride, but that was a myth. The smooth ride of Pullman cars came from a more sophisticated springing system and devices to reduce rocking. This explains why six-axle trucks disappeared with the arrival of lightweight cars in the 1930s.
So if I’ve got this right, when you’re talking about running heavier freight cars on rickety trackage, the overall weight of the car is more of an issue than the weight per axle.
Tom
The same article on weight mentioned developing crossings where the wheel rides on its flange to avoid impact. I knew streetcar lines used this trick a century ago with their light equipment but was always told that regular railroad car flanges would break instead of support the load. About when did freight car wheels become strong enough to support the car’s weight on the flanges?
A lot of this topic has also been covered in the “Cost of rail” posting…
http://www.trains.com/community/forum/topic.asp?TOPIC_ID=58707
I often wonder if the max weight allowable should be determined by the weight of engines themselves. When you consider the weight of an F-45PH, with voluminous fuel tanks, we are talking major weight here!
On the highway, many ‘rigs’ weigh about 3/4 as much as the trailer they are to pull, with full fuel tanks. Hence the reason for the ‘10-wheeler’ bobtail on the road.
Does anyone know the exact “curb weight” to compare, say, the Alco Century locomotives or EMD GP 7 or 9 with the F-45PH? Also, compare to the SD45? Or any other engine with 6-wheel trucks, as well as the ‘experimental’ unit which had 4 axles on the rear, thus gaining 1 traction motor, and 2 sets of additional brake shoes.
I am positive the comparison of lading to motive power would come in 2nd to the motive power!
If the rails are safe for the ENGINES, then they should be safe for CARS of comparable weight / tonnage!
Yer Hillbilly friend in TN…
Underw8
Apparently that was the case in steam days, when the locos greatly outweighed the cars. The article in Trains, however, indicates that it’s not just a matter of the maximum weight of any single car but also of cumulative stress of the entire train – loco plus the cars. Cumulative stress seems to have become more important because maximum car weight seems to be approaching the weight of locos. Clever handle.