To allow yourself a momentary disconnect. To watch the world out of the window, or the people around you. To take an interest in the weather and season. To give your mind a chance to digest what you mean to do or what you have done.
I cherish my time travelling from one place to another, even if that is just a walk to the shops, a trip on the bus or ride on the Underground.
I also like to have 100% focused work time and space. It's hard to find that focus when you're willing to mix work with everything.
If you are always in the stream of data that the internet provides you don't find the time to contemplate if you are moving in the right way on the short term stuff... Most of my best decisions have come from going to wash my hands or just walking outside the office for 5 minutes. Our ever connected world makes it harder each day to see the big picture.
I'm sorry to sound misanthropic or otherwise cynic, but people on the train are, for the overwhelming majority, uninteresting or annoying. I don't take that much interest in the landscape or weather, either. I'd much rather disconnect from that and hack on my laptop instead, thank you.
And they're usually the same people every day, if you catch the same train.
For example... I happen to know that if I take a commuter line into Boston in the morning, I am likely to encounter one or more of: a senior scientist at an R&D lab, a bunch of Google programmers and engineers, a non-profit fundraising expert, or an IP lawyer. I don't know what the guy with the MongoDB sticker on his laptop does, but eventually I'll say hello and find out.
Maybe you should drink your coffee before you get on the train and try finding out who people really are.
I'd rather do those things in an environment and at a time of my own choosing. Work done on train or bus = work that doesn't have to be done at office -> more free time.
It's the same tunnel it was yesterday. And the day before. And last week. And last year. Come to think of it, the tunnel hasn't changed much since it's inauguration in 1984. And the soundwalls lining the surface parts of the railway line aren't that captivating either.
I can tell which station we are pulling into by the sound and the feel of the track! Sometimes, I leave early on a Friday and walk the 10 miles back by canal footpaths.
Something like that.
Point is, I don't feel the need to be laptopped-up and my work/life balance doesn't demand it. Sound's like this was an optional activity for you too.
On a very long train journey, I'd listen to the radio, read the news, watch a video (on the phone's 5.3" screen), talk to people, check my email - do anything I want too.
Reading the news, watching a video and checking email all seem like more pleasant tasks on a laptop than a smartphone of any description. Perhaps we have different preferences.