I sometimes would bring ear-protection headphones that I'd wear over my earbuds to muffle the train noise.

I used an Akai LPK25 with my iPhone (using the Camera Connection Kit and a combined usb hub/dac) and an app called Simply Piano. They make a wireless version of that now that would simplify the setup a great deal. It is a mini keyboard and the keys are quite small, but in my experience it was fine for the beginner stuff (and the keyboard is useful in general later). As I said before, I stuck with this until the app started using keys outside the range I had.

Now, as for "did I proceed with more serious learning" - I alternate though a ton of hobbies. So I moved on after that, though still go back to it from time to time. But I also have other musical interests and it was helpful to those as well.

Also did a lot of music on the commute on my iPhone with Korg Gadget (and Caustic before that). Sometimes with a keyboard, sometimes without.