Do you know that feeling that halfway through the day you start to be less 'fresh'? Myself, I have the experience of getting a bit slower over the course of the day: in the morning, I can fix the problem I was struggling with the last afternoon in a few minutes, but after lunch, problems seem to get a bit harder to solve.
I call it having a finite amount of braincycles, but really it's 'just' biology. Research showed that your brain gets smaller throughout the day and it gets polluted by waste materials and toxins that get cleared away when you rest and sleep (sleep deprivation can even kill you); apart from that, it's also because of the way our sleep cycles work.
These processes mean that during the day, your focus gradually decreases. It makes morning the best time to do work that needs focus, inspiration, imagination and creativity. Meetings then are generally better done in the afternoon, so you don't "waste" precious time where you can get Things Done (that's how it feels for me).
Also, give your head a rest by not staring at your smartphone, but out of the window of your train; let your thoughts run free. Staring off into space or daydream can be good for creativity because "our brain begins drawing connections between all the things that it previously didn't see as all that connected. Importantly, the brain networks that are responsible for creative insight come online." Something to think about :)