@pie i'm gonna go with probably yes because there's likely to be blood and bleeding on the train is a bit rude
@pie if you're changing out piercings for the first time, that means that you're probably not totally healed up yet - they usually recommend to keep it in (but twist it every day) for like a week or so iirc (i'm fuzzy on exact length)
there will probably be a touch of blood doing the first swap of earrings/etc., but a tiny bit. probably more likely a scab.