It appears to have been a 125 and yes, usually there is a loco / power car at each end.Looking at the state of the wreckage, it seems that the train was going at a reasonable lick. According to the circumstances that have emerged so far, there are one or two points that need clarifying.
Presumably the train was a multiple unit (a control cab at each end) rather than a loco hauled train, which enabled the driver to change ends when he came across the first landslip and drive back towards Stonehaven.
He would have had to contact his controller to alert them of the situation and to receive clearance to reverse 'wrong line' back to Stonehaven.
Nevertheless, under those circumstances, he would have been operating under strict 'wrong line' rules that are stiff with restrictions and the permitted operating speed would have been a very slow crawl.
I daresay it'll all come out in the wash.
PS: Anyone want to start an ARRSE Rail Accident Investigation Team?
Hours-long gap between rail derailment and emergency call - reports
Emergency calls were made hours after the derailed train is believed to have gone off the tracks.
However there are varying reports on whether the train derailed shortly after the 20-minute journey between Aberdeen and Stonehaven around 7am, or closer to 9.43pm when emergency services were called.
Train derails with 'two dead' and serious injuries - all you need to know