It is interesting that there is no mention that the vehicle towing was unsuitable for the weight towed and this would have had a signifcant effect in pushing the whole ensemble down the hill on to the track.
Fatigue was certainly a contributing factor but so was the vehicles inability to stop !
All motorway bridges over railways are now protected by concrete parapets following a fuel tanker crashing off the M56 on to the West Coast Main Line near Runcorn some years ago but this vehicle came off a long way before and slid a substantial distance (off road tyres are useless at stopping a vehicle doing 50+ mph on wet grass !)
Whilst his falling asleep was contributory, his stupidity in what he towed with what is even more damning and leads on to the whole debates about caravans etc and licencing of drivers/vehicles.
After all most motorway accidents these days seem to be caravans attempting to rotate without any call of " V1 " or " Rotate" from the driver !