If I have time I always talk to JWs when I see a cult cart.
They are in a public forum promoting a harmful ideology therefore they are fair game. Having said that, the individual JWs on the trolley are not the enemy so I think it's better to keep it as friendly and non-confrontational as possible.
Being realistic no JW is going to admit they are wrong because of something we say to them on the street but don't underestimate the long-term effect of doubts.
So what are your ideas for how to approach them?
Personally I like to use the literature on the cart. If they have stuff on creationism I ask them about that. If they have brochures on why the bible is god's word I ask them about things like slavery and genocide. Make sure you have specific references in mind.
IMO arguing christian doctrine like the deity of Jesus is a waste of time. It's one proof text versus another.
Don't waste time telling them about bad stuff the cult did decades ago. They don't care. The ARC is a powerful weapon but make sure you have some succinct facts so it doesn't just sound like a hateful rant.
The "overlapping generations" is always fun.
So what if they dismiss your challenge as persecution? That's just something they tell each other to pretend it didn't affect them.
One last thing. Don't tell them you are an ex-JW until the end of your conversation. Then tell them how leaving was the best decision you ever made. Try to leave them as a friend with a smile and handshake.