My Spanish wife often mixes up "on" and "in" when speaking English because there's a single word for both in Spanish - "en". As well as standing "on line" like New Yorkers, she often "bumps onto" people - because "you don't go inside the other person's body".

Prepositions is an inheritable flawed concept in all languages, like on the train, on the station, in the square. It should ideally be replaced with some transport inflection and lacking that just ignored when it is used for common phrases and not actual space positional markings.

A line is one dimensional, to stand "in" line implies more there is more than one.

Did a brief stint at our London office in the 90s and had a long debate with one of the locals about 'on' vs 'in.'

I thought that was a Jersey thing. Never came across it until college when a Jersey person mentioned it. Sparked a nice debate around terminology and pronunciation in the group (otherwise made up of MA and CT).

Well, are you standing on the line or have you been spaghettified?

I think it’s the same idea as being on a team, a board, a committee, etc, and not in one — on denotes membership.