Re: Arcane question of the week

Jack Mullen


That could be the arcane question of the month, if not the year.  As I'm sure your are aware, this isn't readily found.  Some values can be picked up in reading track-train dynamics studies and derailment analyses, most of them well past steam era. 

Currently (for those of you whose calendars don't turn past 1960, that means WAY past steam era) AAR interchange rules limit combined car+load CG height to 98".  It's been remarked that it's not possible for a loaded Plate C boxcar to exceed this value, so if you assume a load mass equal to the LD LMT, located 1/2 IH above the floor, you could derive an upper bound for the car's CG - for a modern car of course. And UMLER now has a field for empty CG height,though it's only a required entry for certain cars.

Looking back to the steam era, or closer to it, at least for steam-era equipment., I can offer a few random tidbits. My references are packed for moving, so this is dimly recalled gleanings from the distant past.

For a flatcar, something like 3' or a few inches  less should be a reasonable value.  For a boxcar, IIRC the empty CG is generally within a foot or less above the floor.  Having gone through the rock-and-roll derailment era of the '70s and '80s, I think large covered hoppers tended to have an empty CG around 5', and loaded around 8', and both values were regarded as higher than average, so again that may help with an upper limit.

For steam locos themselves, as a rule-of-thumb, the height of the CG should be around the bottom of the boiler.

It's not clear to me whether you're talking about tipping to the inside of a curve in a stringline derailment, or overturning outside in an excess-speed derailment. Since speed is mentioned, I think it may be the latter.  Empty-car CG should be low enough that overturning shouldn't occur at any plausible combination of train speed and curve, so if the model is predicting otherwise, something is wrong.  In practical terms, it's likely that an L/V derailment, either wheel-climb or rail rollover would occur sooner.

Jack Mullen

Dave Nelson asked:

How far above the railhead would you say is the center of gravity for your
average empty STMFC?


Our software team has added curve resistance to the rolling resistance
values and while doing so tossed in something else for tipping over at some
relationship of curve radius, super-elevation, and speed. I think it's far
too sensitive but I need a decent center of gravity estimate to make the

Dave Nelson

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