When I used to have a long train commute to and from work, I used TDD as a way to manage context switching. I'd write the test before leaving work, make it pass on the train, write the next test. Then, the next morning on the train, I'd pick up and make it pass and have the next test queued up before the train arrived. Once I'd walked from the train station to my desk, I was all ready to go to make it pass.