Well, yeah, there's nothing wrong with posting off-topic. But I like the challenge of writing on-topic. The topics here have sent me off in a lot of interesting places, and I think the better things I've written for 600seconds owe maybe as much to the topics as to whatever weird way my brain works. Even something like my neverending honestly-I-haven't-forgotten-it-even-if-you-wish-I-had story, which is about as off-topic as it gets, owes its genesis to the topic that was posted the day it started.
Writing on the train would probably be more trouble than it's worth, to be honest. I often spend the morning commute standing, and even when I get a seat, the train's pretty crowded. A book in my lap is about the best I can manage.
But there is that lunch hour. I just need to find a news aggregator that does send feeds to e-mail. Mine doesn't appear to.