I will admit that Walter blowing up a building and stealing chemicals from a high-security facility while setting the alarm off may have pushed the limits of my willing suspension of disbelief.
Agreed, that was way worse than the train robbery. In fact, it was so unbelievable that they didn't even show what happened in between the alarm getting set off/the police heading toward the scene and Jesse/Walt/Mike getting away.
But when I compare this show to just about everything else I watch, I really can't sit and complain about lack of realism. A lot of outrageous things happen, but this show does a better job of anything I've ever seen at making things seem real
What I think really helps is a lot of the small stuff, namely the dialogue.
The little things like Mike's lawyer having a random conversation with the bank attendant about cookies - they do them so well and I really feel like I'm watching something happen in real life right before my eyes. There's something really organic and unforced about it.