I have to admit this one is very confusing and I am not usually a fan of trial lawyers.
If someone pays the fare and strolls down an icy platform or one that has deteriorated in some fashion, they are out of luck since they did not board the train at that point. The owner would not be responsible.
I suppose there is a slight difference in that BART itself did not employ or otherwise encourage the offenders, however that doesn't seem prominent in the logic. The decision supposedly states the victim is not entitled to protection unless physically on the train.
Somehow I think if there were another entity involved, the decision would not have gone this way.
Just IMHO.