If you get paid by the hour and you're required to be in an office during a set period of time, then yeah, I hope you aren't working for free on the train. This is far from the only situation though...

I guess I'm lucky enough that my managers have never hounded me about when I come in and leave, so if working an hour on the train means I get my work for the day done an hour early, it also means I leave the office an hour early. Yeah, maybe I could use that hour to read for fun instead, but I'd rather finish everything sooner and get on with my day sooner.