Let's dissect the question into its two functional parts: inertia and friction.

To keep it sweet, let's say we have a five-pound bag of sugar, sitting on a roller, atop a 30-degree slope. Now, if my computer can calculate that angle, let's say that it is somewhere close to one-third of the ninety-degree slope that results in 32 FPS^2 standard acceleration at 1-G. Let's abbreviate that rate of accceleration, G/3. In short, at one foot of accelerated movement, the bag is moving at 10.666~ FPS. That yeilds 53.333~ foot-pounds inertia.

Now, let's up the ante just a hair,and say we have a train, loaded to 250,000 tons gross weight of boxcars (five million pounds)packed with sugar, and it's going down an impossible grade of thirty degrees.

How fast is it accelerating?

Exactly the same rate. Only it is one million times more difficult to stop: "Casey Jones, you better watch your speed," sang the Grateful Dead.

http://en.wikipedia.org/wiki/Acceleration

http://en.wikipedia.org/wiki/Inertia