Glasgow's Evening Times is reporting that some of the rear wagons derailed, then hit and brought down power lines, and then an underbridge collapsed...
That's got to be an early contender for Scottish News Picture Of The Year...
EDIT: Please correct me if I'm wrong, but looking at the news photographs and video, this incident seems to have happened in the vicinity of a Network Rail work site - there seem to be new track panels nearby, and that temporary blue fencing stuff, and lifting equipment already on site... but, please, correct me if I'm wrong... (images from Evening Times):
FURTHER EDIT at 1612: BBC Scotland's Lunchtime news report suggested that the bridge, which was due to undergo work next week, collapsed first...
http://www.bbc.co.uk/scotland/player/?item=44534828