J.W.
I don't think you are being paranoid but what may have been at play may not have anything to do with age.
Have you ever driven on autopilot? What I mean is you get in your car to go somewhere, you are thinking about everything you have to do that day and you find yourself driving to where ever you go the most and not where you actually wanted to go? When my older kids were in a private school that is where I headed, now that I work I always head to work. :-/
That may have been what was going on and it has nothing to do with age. There is a pattern to street lights and even trains. When you are in auto pilot unless the light usually turns red at this point you will actually run red lights without even noticing. It would have been just bad luck that it was a train and not an intersection.
I know I am not the only one to do this. It was a regular topic of conversation when we were waiting for our kids to get out of school.
Oh, obviously since I consider this normal, absent anything medical, I would let her stay with grandma. My mom had Alzheimers and it was very obvious when she hit the point she should not be driving herself or anyone else around.