I have to do the "long time lurker; first time (in ages) poster" thing (I always follow and love everyone's comments on the episodes!) to raise a couple of things:
1. I think everyone complaining about Kate's career VASTLY overemphasizes how big a deal "curriculum development" is. Her getting her undergrad and masters degree in the time allotted is a bit of a stretch, but curriculum development is usually done by committees, and a competent and passionate educator with a strong interest in curriculum, especially in a specialized field like music education for the blind, could easily end up on committees designing state curricula ... and if it's something she's good at and interested in, it doesn't seem a big stretch to me that she could be part of a group presenting on the curriculum they developed, at an international conference or something. To me (probably because I've worked my whole career in education) it seems much less of a stretch than Randall going from city councillor to state senator in the same period of time, and it's never "shown" how he did that either -- seems odd to me that people are so puzzled by Kate's career moves and not by Randall's.
2. In the minority here, I liked the Marcus storyline. It's something this show has often done -- taken a character outside the story, teased us to as to how they link into the main story. Sometimes (like the guy who lived with Randall's mother) it's a character who ends up tying into the main characters' personal stories, but other times (like with the guy who developed the technology that allowed them to use videocalling during the pandemic!), it's a more tangential connection that touches on our characters' lives. This was kind of both, since Marcus had that hospital connection where the doctor saving his life missed saving Jack's (I did question why there were so few doctors in a big-city hospital), but also the Alzheimer's thing. I didn't think Marcus was going to "cure" Alzheimer's; I assumed we were supposed to conclude that his research had contributed to developing the drugs that gave Rebecca a few more good years with her family, as seen last season and earlier this season.
3. The train metaphor was lovely but the discussion over Jack vs Miguel on the train really emphasizes the problem with a very literal view of the afterlife when it comes to people who have been widowed and happily remarried. You either have to imagine a sexless afterlife or a polyamorous one -- a philosophical conundrum people have been wrestling with for at least 2000 years if the Bible is anything to go by. I thought from a storytelling perspective, the years with Miguel and the depth of their love for each other got due attention in the Miguel episode, but Rebecca's deathbed scene was definitely going to bring her back to her first love and the father of her children.
