For what it's worth the subway is very safe in terms of violent crime. But pickpockets are a scourge yes. The problem is the police can only give them a fine if they steal less than 400 euro in value (see more info about that here: https://www.theguardian.com/world/2011/jul/14/woman-defends-... ). Even if it's their 5000th time. There's gangs of professional pickpockets.

They got me once and I've busted them trying a couple of times. Whenever you grab them they always just drop the phone or wallet and walk off, they never fight. But I did notice they always work in groups so they probably do attack if you get violent yourself.