That's a Gloria scene. In fact, I'm not sure how you got the one
after it without getting it first.

All of her scenes on the subway should trigger just by riding the subway enough times, after having watched the news report about train chikan. That shouldn't be possible, but ... my best guess is, maybe the timing of when it should have appeared just so happened to line-up with some other event that triggers on the subway, and it got skipped over.
Good news is, gallery unlocks are game-wide. So, assuming you save fairly often, I would recommend going to a save somewhere around her first scene on the train, and then just spam riding the train until you get the one that's missing.
Going to the beach (when there is no event there) is the fastest way to spam train rides, because it kicks you back to the train when nothing happens. So, you can hold down the skip button and just choose beach repeatedly until you get enough train rides racked up. Should take less than a minute.
(Sidenote for Nikraria:
IF I'm right about that, you might be able to fix it by just making any events which occur on the train but are
not dependent on the number of train rides subtract 1 from the "number of train rides" variable.)