The discussion was about leaving unattended valuable objects in public places. Sure, a theft can happen even if attended, or using violence, but I personally avoid increasing the chance of having something stolen by leaving it unattended.

If I would make a statistics of primary cause of remaining without a laptop among people I know, the biggest danger is liquids in glasses (that ends up on the laptops) ...

A random person able to dart in and then make a getaway is not what "working on a train" is like and that was the original comment's point.