Not the runaway train

Here's the train going back down the hill at Threlkeld quarry. It was pointed out to me that the carriages would naturally keep rolling downhill once moving under gravity. There is friction between the wheels and the rails. This is equal to coefficient of friction x normal reaction force. There will be a large normal reaction force from such a heavy carriage. But provided the component of gravitational force is bigger than this, the carriages would accelerate downwards. The problem of friction brakes is that they get very hot if constantly applied and can burn out. A car's petrol engine can be run downhill in a low gear to have a braking effect but this is harder to do with a steam train although I have found articles about "countersteam" dynamic braking. It seems a complicated subject so more thinking needed.