"Oh, hey," you whisper, leaning towards a woman dropping her drink bottle on the floor. She's ignoring you. "Oh, hey... I didn't realise this was the RUBBISH LINE." She ignores you again, clearing the burger wrappers from her pockets before alighting. Just take your rubbish with you, jeez.
(Of course, you didn't actually say anything out loud. However you did draft a really disparaging tweet, and that's what really counts, right?)