Well, that might depend on the commute. I'm on the train from 8:20-9am, and I've not yet had coffee, so there's no productivity to be heard of there, and at 5:20-6p the productivity tank is definitely empty.

Perhaps the morning side could, if I were awake. Although, on the afternoon commute, I read stuff such as on HN because I am mentally spent. (Though if it's too rigorous, like a deep CS paper, that might get passed over.)