"Ladies and Gentlemen" beyond binary gender classification

You can use everyone, which is what the London Underground now uses:

Underground staff have been instructed to begin their announcements with phrases like, “Hello, everyone” and “Good morning everyone.” The transit organization Transport for London (TfL) writes that “Ladies and Gentlemen” has also been scrapped from pre-recorded announcements.

Mark Evers, director of customer strategy at TfL, says in a statement that the change was made because the organization wants "everyone to feel welcome on our transport network.”

"Ladies and gentlemen" is a phatic expression, that is:

communication which serves a social function, such as social pleasantries that don't seek or offer any information of value.

If you're going to cut hairs on even that phrase, then you should be saying

Ladies and gentlemen and boys and girls and teen and preteens and elders and babies and deceased and conceived and...

But we all have things to do with our lives. So we just say "ladies and gentlemen." (Now, people who don't have things to do might wonder about that expression and why it isn't more "inclusive," but I digress...)

As to the context of your question, if it were on a train, the announcer could simply say, "Attention passengers."

Boom, done. Next.