Kind of an extreme example here but if you were to build a train track tower on the west coast 10,000 ft high. It gradually sloped all the way to the east coast about 3,000 miles. Only dorps about 3 ft per mile. Would a train start rolling on its own with perfect wheels, if so how fast could it accelerate to? My first thought on this is that at this low grade wheel friction would be the dominating factor.
Friction would stop the train from rolling.
If you could, in fact, have “perfect” wheels – perfectly round, and frictionless – then, yeah, it would slowly accelerate and roll down the hill to the destination.
(Actually, I’m not at all sure what the breakeven grade is for modern RR equipment. Three feet per mile? Ten? Fifty? I don’t know.)
This site tells of the steepest US railroad grade. “The steepest adhesion railroad grade in the USA is found at the Cass Scenic Railroad in West Virginia. Shay geared steam locomotives haul tourist trains up a maximum grade of 11% on this former logging railroad.”
This site gives some physics background, and offers a value of the “rolling resistance coefficient” for a typical passenger railroad car as between 0.001 and 0.0024. Ordinary car tires on concrete score 0.01 to 0.015. I think this means that a railroad car will roll downhill on a grade approximately 1/10 as steep as a grade where a car will just stop rolling.
(More accurately, where the tangent of the grade is 1/10 the tangent of the grade where a car just stalls from rolling. But at very small angles, the tangent becomes linear with the grade itself, so “1/10th the grade” works well enough.)