Guess I'm a bit hypersensitive today, cause I found this one a bit irritating. The idea that a straight girl has to be concerned about being closed in with a group of lesbians triggers me a little. It's like suggesting that lesbians are predators constantly on the hunt for fresh prey and will pounce when she wanders into range (never-mind that one did), even if just for comic effect. It felt unpleasant.
You're overthinking wrong. See the lesbian only car is a play on women only car. With a straight girl in the L-only car being analog to a guy in a woman only car.
Basically this would be a wagon where Lesbian can be as gay as they want in peace without getting judgemental looks or comments from other passengers. (a bit like Gay bar or club, but with a company mandated exclusivity rule).
Heck if the issue was fear of predatory lesbians, pretending to be one would Not help the protagonist, since it would just make all the lesbians think she's also a lesbian, and thus open to the idea of having a relationship with a woman.
She's just concerned about being sanctioned for breaking the rule.