Hokie 71, I understand your interest in the string lining issue.
It's true that string lining can occur, however it has to involve degree of curvature and trailing tonnage and drawbar force. The degree of curvature of the typical wye track is sufficient to induce string lining if the locomotive is pulling hard and there is a lot of tonnage behind empty cars. Certain types of cars and certain train makeup conditions are a factor also. Long car-short car coupling configurations can be a factor. And, as we have learned from subsequent posts that N&W 611 was pulling its own weight and the gondolas were partially loaded. And you can bet that a high profile, high value load like a steam engine will not be handled without some supervision other than the operating crew.
Whenever you see a string lining derailment, you'll normally see a train of some length with a locomotive on the head end, pulling hard, producing high draught forces through the draw gear. That can also turn the inside rail of the curve, if tie and spike condition is not good, in which case a car can derail and dig in, since it is no longer on the track, and, if there is an empty car, especially a long car, ahead of it, that car can jump the rail.
Also, the Engineer has some involvement. The lack of prudent throttle handling, whether through inexperience or through just not being one of the best Engineers, or through just not sensing that conditions are ripe for excessive drawbar pull, is often the root cause. That would never be allowed on a closely-watched shipment like N&W 611.
So, your question was good. But more severe conditions are needed before string lining could likely occur.
