That may be for a free falling weight. But we try to avoid our trains from free falling. Put two cars on a grade. One heavy and one light and let them both roll and see which one goes farther and faster. Then get back to me with the results. Just like the Hot wheel cars of our youth. The heavier car always went faster and further. Neither hit terminal velocity like a free falling object.
But kinetic energy (KE=mVsquared) and momentum (M=mV) does.
Our models have very little mass and very little velocity, they have very little kinetic energy and very little momentum.
A quarter pound model boxcar isn’t going to perform the same as a 260,000 lb. prototype boxcar, a prototype car moving at the same velocity as a model boxcar has a million times more momentum and kinetic energy.
I built a pair of 30 foot gun flats from F&C. I used trucks and wheels from a pair of H21 hoppers. The hoppers would roll away on any slight grade. The flats without load will sit still on a 1 in 12 grade. Once the load is installed and the spacer flat in the center it rolls a lot easier once started. Getting over the initial starting friction the mass overcomes the friction by momentum. It’s the added mass that keeps it going.
Having rode over cars in a hump one must factor in the damage to payload and car if going to quickly.I think O scale with metal wheels might work out better