Honestly, if you have a residence of some kind and an Internet connection, you don't need to bring your beefy computer with you everywhere. It is cool to be able to have ridiculously powerful mobile computers, but I don't think I would ever be willing to take a $6,000 laptop anywhere it has a decent chance of being stolen.
You can use a desktop computer on a train if it's one with power outlets. Might get some funny looks, but I've seen it happen (or at least pictures). :)