Before...
This is why, when you first get to the gym your heart rate has been at stable rate, it's going to take about 10 mins to really get your body in fat burning mode.
But if you wait until after you've trained you go right into fat burning mode.
Why?
When you train (weights or cardio or both?) you burn fat, you train for about a hour, that's a hour worth of fat burning you have before doing any cardio. You sweat more, an burn more fat if you wait until after your training session. (So shouldnt the answer be after?)
About the waist trainer's, the only example I have is my wife. When we started she was 146ish at 27.5 inch waist, right now she is at 141 lbs at 25inch waist in 3 month of training, she has to buy a smaller size waist trainers because the one she used got to big.
You can definitely lose on the waist without waist trainers but normally to lose a inch or 2 you have to drop 10-15 pounds, or I least I do.