Steel vs. Aluminum vs. Stainless Steel

Hi folks, what are the relative merits of rolling stock with aluminum, carbon steel, or stainless steel bodies? (Applies to both passenger and freight stock both.)

Well, to get the discussion started…
You need a certain amount of structural strength to the carbody. Within limits, if the desired structural strength is achieved, the lighter in weight the better. (Limits: you wouldn't want the car to be so light it would float off the tracks when empty. For practical purposes we can ignore that danger!)
Aluminum and stainless steel both allow lighter weight for a given strength. I'm not sure which is better in this regard: aluminum is lighter than (a comparable volume of) stainless steel, but stainless steel is stronger than (comparably dimensioned) aluminum components. My ***impression*** (from random reading, not systematic study) is that, at least for passenger cars, the lightest tend to be alluvium, but that well-designed stainless steel cars come very close to them in weight.
On the other hand… Aluminum and stainless steel are both much more expensive than carbon steel.

Now things get complicated by operational factors. In loose-car freight railroading, freight cars are in actual use for a shockinlysmall proportion of the time. So economically you don't want to spend big money for small improvements in efficiency. But in unit-train service the cars (typically hoppers…) get enough better utilization that at least some hopper-car purchasers have felt it is worth while spending a bit more to get lighter (empty) weight aluminum hopper cars, which (since the maximum weight when loaded is governed by things like rail and bridge strength, and so are the same for hopper cars of different construction) means each car can carry a bit more payload.

Now: I'm sure there are people visiting this forum who know a LOT more than I do (and who can give some actual NUMBERS): please educate ME (and give a better answer to Mtuandrew)!

Carbon steel has a disadvantage, it rusts.

This is what P-S cars ordered for the Morning and Noon Daylights looked like in 1940 or so, as they worked in service LA-SF.


Here is a pair of articulated coaches sometime after 1960. They had rusted out, from the salt water of the Coast Route. The frames were renewed by the Sacramento Shops with stainless, and stainless sheets became their exterior.

Aluminum can handle big bulk, rest assured.

Sadly, none of the images of a coal gon I want to post are working.

I cheated when I posted, I already knew that both aluminum and stainless steel have much more corrosion resistance than carbon steel* and that stainless steel can approach aluminum in terms of strength per pound over a given area. Both materials have fabrication costs that don't apply to carbon steel as well (welding in particular.) What about their relative durability and relative cost? I'm curious why (for example) Amtrak, NJT, and Metra have specified stainless steel construction for their rolling stock since the Comets and the old P-S gallery cars respectively, but other agencies are still willing to buy aluminum cars. Aluminum coal hoppers I think I understand, since they carry a heavy, soft load that weights out before it bulks out on 283k cars (not sure about 310k cars.)

* Excepting Cor-Ten Steel, which forms a thin layer of surface rust in order to stop further rust. In railcar use, it hasn't been too successful - see the Metra Highliners.

Thanks, John!
Random remarks:
--CorTen steel is not new. I believe that some of the New York Central's J3 Hudson locomotives, at the end of the 1930s, had CorTen cabs, and others aluminum. (The J3 was not a heavy locomotive by the standards of, say, the Chesapeake and Ohio, but the New York Central (perhaps??? worried about damage to track from locomotives of fast trains???) evidently wanted to shave off ounces: CorTen and aluminum both allowed lighter structures than conventional materials. Or so I think I remember reading.
--As for Amtrak's preference for stainless steel. Amtrak runs trains in areas subject to salt spray: the aptly named "Shore Line" of the New Haven Railroad being an important one. I recall reading that, when Amtrak was established and its technical people were inspecting cars for acquisition from the private railroads, they decided that aluminum was more subject to corrosion than stainless steel in these environments.

That makes sense. I've seen some very nasty things happen to aluminum when exposed to even small amounts of saltwater spray.