I was one of those people who was upset/confused about why Miguel got got so little time with Rebecca on the train. Someone on Reddit posted their view (below), and it did make sense to me:
I feel like a lot of people are missing the fact that all of this was going on inside Rebecca’s head, in the mind of someone with Alzheimer’s. In a way, she’s her own unreliable narrator, and her recollection of the last few years of her life is unfortunately non-existent. She’s not the late 70s-aged version of herself; she’s Rebecca at her prime. All of the people she sees actually manifest as they would have been at that time. Of course Miguel gets short shrift - he was only ever Jack’s friend in the time frame in which she sees herself. The big three appear as adults, but her flash of recognition seems more palpable when she sees them as children and young adults. William symbolizes the confusion and self-flagellation that came with keeping secrets from Randall, and in her last moments, he (and the doctor) permit her to forgive herself for her life’s mistakes. I think his was incredibly well handled. I’m not sure if people really were expecting the metaphorical “life flashing before her eyes”, because what they got was something significant and profound: a woman, at the end of her life, forgiving herself and being forgiven.