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Bum-rushing is, IMO, strong, but the problem with striking up a conversation with someone in a line, is that they're more or less a captive audience. If, for some reason, they'd prefer not to speak with you, they have to leave their place in order to do so, and forcing that on them isn't really polite, or fair.

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Or they communicate via their body language that they aren't interested in a conversation at the moment which you read, understand, and respect by ending the conversation.