I’d like to put Rails logo on my business card so that it looks clear that I like Rails. However, I believe Rails logo is not public domain. Could I do that?
If you read Ruby on Rails — Trademarks policy you’ll see that it will depend on how you phrase it. You cannot use it to show some form of affiliation, but you can use it to show support.
Since the usage in you specific case does not seem one or another, I would probably (and because I’m not a lawyer) contact the Rails Foundation. Ruby on Rails — The Rails Foundation
Hi Masafumi, Feel free to write to us at the Rails Foundation about this: email@example.com Amanda