I've been lucky so far and haven't seen that yet in T:ANE, but as said here I've had that occur in earlier versions. Hehe.. I had a freight consist roll away down the main off the top of a spur.
How steep is the grade?
Some freight cars have a setting in them to allow you to set the brakes. Click on the ? button and check the properties.
Unfortunately, this may only be available with specific assets by Jointed Rail which I'm sure wouldn't be found on a route set in the late 19th or early 20th centuries.