An event that is so well-attended that all of the chairs in the venue are occupied, leaving only flat spaces of pavement or flooring for other attendees to stand.
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Public transportation services that have extremely high ridership.
Wiktionary
Standing-room-only Sentence Examples
Despite the pub's standing room only capacity, the table next to her booth was empty.
Her first top-five single, in 1975, was called Standing Room Only.
Even with a press pass, standing-room-only is often the only "seat" in the house!
