Bombing trains where im from goes through its stages, sometimes you get on a train wiht 3 people on it, two hero's and a cop, so you dont touch it. Others ive been on and thrashed during peak hour with people everywhere, jumping over people from wall to wall without a word, even with ink going everywhere.
Street is easy, never gotten into it heaps but when i do all you gotta do is take a few seconds to think, look around as your walking up, you see someone giving you the mean eye then dont do it. With cars, dont let them see you wakling up and starting, but if a car happens to drive passed, keep going and finish, then walk off rather briskly.