Two trains packed with commuters crashed head-on in Vienna after a railway worker apparently forgot to activate a signal.
More than 40 people were injured, five seriously.
OBB, Austria's state railway, said initial investigations showed that a supervisor neglected to trip a signal after manually activating a rail switch. The switch usually works automatically and sets off the signal but was stuck due to snow and ice.
The accident occurred with the two trains heading towards each other directions on a single set of tracks. The signal would have normally warned one of them to wait on another set of tracks until the other passed.
The railway said the accident was apparently due to "human error of the responsible supervisor."