It was pretty obvious from the moment Jeremiah resurfaced earlier this season that he wasn't actually a prisoner of Cadmus as Kara, Alex, Eliza, and everybody else believed, but was in fact working for/with them, making the show's attempt to convince us as an audience otherwise kind of pointless. However, to the writers' credit, they actually managed to make what would've otherwise been a needless misdirectional cliche work for the most part.
I continue to really like Mon-El, both individually and in his relationship/interactions with Kara, and so I got an immense kick out of the show revealing that they'd consummated their feelings for each other. I also, like Christopher, found it funny that Kara's response to (presumably) losing her virginity (since her Kryptonian biology and its responses to Earth's yellow sun would've made it impossible for her to have sex with anybody whose biology wasn't also affected by Earth's yellow sun) is to go out and be Supergirl in "overdrive".
Given what ended up happening with Jeremiah and his true allegiance(s), his and Eliza's conversation takes on a whole new meaning and tone, leading me to wonder if she too suspected something was 'off' about him but chose to give him the benefit of the doubt and not say anything once Mon-El "shot from the hip" and went off about his suspicions. I also half-expected Kara to use her super-hearing to eavesdrop on Alex and Eliza's conversation towards the end of the episode, especially since she would've been within her right to do so given that she IS part of the family officially, but it's also okay that she didn't.
I love that we got to see Winn and Lara (Lyra) having some fun and just being all lovey-dovey and 'schmoopy', and thought it was funny that Winn didn't even bother to formally introduce her to Mon-El, or vice-versa, since he was obviously too smitten to really care.
Kara saving the train was a fun sequence that let her use her powers for something other than fighting things, which we don't really get to see a whole lot of; plus, it led to a sequence that reminded me of Barry letting Snart walk away in The Flash Season 1 and Kara letting Livewire walk away at the end of Episode 10 and that let Chyler Leigh deliver a really powerful silent performance.
After the issues I had with the show undermining Alex's coming-out journey last episode, it was nice to see some "supportive Sanvers" scenes, especially at the end when Maggie pulled an intervention to stop Alex from drinking herself into a stupor and then just held her, especially since the writers dovetailed and mirrored it with Mon-El doing basically the same thing for Kara (minus the part about pulling an alcoholic intervention). I also love that Kara once again had her glasses off for the entire scene, just as she did for the scene in last week's episode where she and Mon-El apologized to each other and then made out.
Next week's episode looks really intense, and I can't wait to see what's going to happen, especially with Alex and Kara breaking the rules in order to find their father, Lillian, Hank, and their kidnap victims. I'm also interested to see what the episode's title, "Exodus", ends up meaning, since it's not immediately clear just from the preview.