The train was stopped a mile or so outside the town. It sat on a 1.5% grade which would appear flat to the the eye but is a lot for millions of pounds sitting on steel wheels on steel rails. Did not take long for it to gather momentum as it approached the curve in the village.
Because of a small fire, someone had shut down a locomotive engine not realizing it was supplying the air pressure to the brake system.
Like the SF aircrash.