Nothing is known yet, but there is some speculation as to how, in which she had something to do with it, although only by accident, made possible by some spectacularly bad decisions made by others...
1: the guy who is responsible for bringing the train to and from maintenance (or something, I'm not entirely sure about job responsibilites in rail yards) left the keys in, engine running, and emergency brakes disconnected/non-functioning.
2: Dead man's grip disconnected for some reason (confirmed by investigators)
3: cleaner accidentally hits switch/button/lever that starts the train, does not realize this until too late. Maybe she thought the guy mentioned in (1) had started the train.
I got to ad that I feel incredibly sorry for her. Just two months in on a badly paid job, and she get seriously injured because of laziness/complacency/comfort.