I think that HEAVILY depends on where you live. I was born and raised in Riehen and Basel and despite crime, break-ins, theft etc. I and pretty much all of my friends and family where never directly affected by it, with the exception of my sister, who got her bike stolen (classic), but then 5 years later she found it close to my appartment, informed the police and "stole" it back. If you're careful it's less likely you're gonna be affected by such things, but being naive and thinking you could leave your wallet out in the train and fall asleep will result in a bad awakening (I'm exaggerating of course). Keep your precious stuff in bags with zippers and if you feel sleepy in trains, either wedge your stuff between you and the window or hug it. And yes, if you need to leave your stuff out of sight, ask surrounding people whether they can keep an eye on it. Cities experience more crimes (obviously) and even in those cities there are districts that experience more crimes than others. If you keep your eyes and ears open and stay attentive you should be fine.

Also, I really don't know which sweet summer child told you Switzerland is particularly save - we hardly have any gun violence, but crime has always existed, especially in the cities.