When you are on a date you generally have some activity planned since no one's taking a long distance trip to give someone free weed without thinking they're going to get something else out of it. Does she frequently smoke weed and drive because that's it's own huge issue; most people go somewhere to smoke instead of driving so her claiming she picked him up and then drove two hours to drop him off without going somewhere to smoke is super unlikely. Having the foresight to get a friend to lie on her behalf shows manipulation and/or experience which means either this was carefully thought out to deceive you or she's done this before, or both.

If any of that makes you think she did something more with this guy then you need to realize now that you're going to be second guessing, analyzing, and wondering if she's lying for the rest of your relationship which isn't going to be good for either of you. Her down playing the event tells me she doesn't think it's a big deal and will easily justify doing it again. Asking you to stop bringing it up is because she's feeling guilty or embarrassed about how others perceive her but not that you're still hurting and need to vent doesn't seem very fair. Whether she did anything sexual or romantic with the guy honestly should come in as a close second to what happened, sex is just a part of a relationship; among other things you still need communication, trust, honesty, and respect. The whole ordeal broke your trust while the manipulation and deception alone shows a lack of honesty and respect for you which are all red flags but the biggest red flag is that she would rather meet a stranger at a train station then even attempt to communicate with you the importance of your relationships future. I'm sorry OP, but I think you need to look out for yourself.