Firstly, you'd be very surprised how a train can come up behind you and you don't notice until it's close by. It seems counterintuitive but I've seen it happen (and had it happen to me in my youth) more than once. But more to the point, she had five children in tow, and when you need to get five young kids to do anything, you can easily lose track of one or more for a few seconds, and that's all it takes to end up in a position where some of the kids are on one side of a moving train and some on the other. While you'd think it's not a big deal, a lot of sober people get killed walking on train tracks every year, so I agree that this is one for the police.
I developed a fear of trains in 5th grade after the Santa Fe came to our school and made a presentation about how dangerous they were and how we should stay away from them (the SF tracks ran the length of the town, 1 block off Main Street, and right down the middle of a street in the business district. Their trestle was the quickest way for kids from my part of town to cross the river to get to the town pool.) I figure the SFRR wouldn't have gone to the bother of making a film, sending staff out to show it in school assemblies, and creating a truly terrifying comic book illustrating the many ways people are killed by trains. Took me a LONG time not to be frightened by trains after looking at that comic book.