Hello, Herb -
Thanks for pointing out the leverage factor, which I had overlooked. In the photos posted several days ago on the GeoPolitics site, it sure doesn't look to me like the rails had spread, or that the ties were all that bad before they were damaged by the derailment. And if the rails were spread, wouldn't the engine have just fallen between them, and not jumped to the outside?
This is just conjecture by a semi-foamer with a little bit of railroading and civil engineering experience, but what seems to me to have happened is combination of at least the following three factors:
1) The leverage that you pointed out, much greater with a short wheel base two-axle engine having the couplers a good way from the wheels than with a longer wheelbase four-axle engine, would tend to pull the rear axle toward the inside of the curve and swing the front axle toward the outside.
2) The couplers are located somewhat higher than the centerline of the axles, so the weight of the train dragging on the rear coupler would tend to increase the load on the rear axle and decrease the load on the front axle.
3) The flanges were probably a bit dry as well - more likely on a sunny day and if the curve hadn't been greased in a while - further adding to the friction and leverage tending to lift the wheel.
With outside front wheel being swung to the outside, the friction on its dry flange would be even greater than when the same engine takes the same curve running light. I'm guessing that the front wheel probably just climbed over the outside rail and then the rear wheel followed.
Good thing the guard rail was there!
If it were up to me, I wouldn't be using a short-wheel-base, two-axle engine on sharp curves with a heavy train. It will be interesting to see what the FRA has to say.
- Russo de los Cinco Fossillos Viejos de Vapor
