The rule
When someone near you is struggling to get a bag into an overhead locker, or a case up a flight of station steps, offer to lift it. Ask first, do it, and go back to your seat. You don't need a thank-you, and you don't need to mention it again.
Why it matters
Plenty of people can't lift a heavy case above their head, and most of them would rather not have to ask a row of strangers. Everyone sees who stands up and who studies his phone. Small courtesies in public are the most honest kind, because you do them for someone you'll never see again.
How to do it well
- Ask before you touch anything: "Can I give you a hand with that?" If the answer is no, smile and leave it.
- Lift it, put it where they want it, and offer to help get it down again at the other end.
- Do it for anyone who looks as if they'd like the help, whoever they are, and expect nothing for it.