Yes, Jade and I are taking Caltrain. CF is right next to the SF station so it works. I have yet to find out if spending 1.5 to 2 hours on the train every day will drive me crazy. :)
I started commuting on Caltrain (Mountain View <-> SF) 3 months ago. For me it's quite easy to read a book on the onward trip and work on the return trip. The only time I'm annoyed is when I board in the morning, people never stand in a line (like BART). They just approach the coach from all directions and pile inside. But it's empty enough that I can wait till everybody gets in, and I will still get a free seat/comfortable corner.
I did Santa Clara -> 22nd a couple years ago when I had a cofounder who lived in the city. I found I ended up writing a significant amount of code for that startup on the train. It actually worked out pretty well; I'd put the finishing touches on features on the way in, then discuss with my cofounder, figure out what we needed to do next, maybe talk to some potential users in the city, and write the code for new features on the way back. If you've got most of the reference docs for the frameworks you're using downloaded or memorized, or are working on stuff that's mostly thinking and not looking stuff up (I'd imagine capnp fits in that category), you can get a significant amount of work done.
That's a very good point. Working from train often means spotty internet connection. So it's hard to instantly look up something in Google. A good offline documentation helps. For older languages and IDEs like C and C++ and Visual Studio 2005 like 10 years there were CHM help files for everything. Nowadays almost everything is available almost only as web help, if there is even an official full documentation at all. Also like 10 years ago, books came with an CHM file on CD/download. Nowadays books are already outdated when the get published, things like App and Web development is moving very fast. So starting a new project on train is not a good idea, fixing bugs from error/warning messages isn't a good idea, but doing some maintenance/refactoring work is well suited for boring train rides.
It's not absurd to me anymore, I "get" how CA, and the bay area especially, operate now :) I've heard good things about Verizon, and I personally use a Project Fi phone as a hotspot. It's decent enough for hackernews/reddit/google/stackoverflow, except for the tunnels right outside the city.