I'm pretty sure this is in reference to the his project that he was wrapping up at his old job.
He is saying that even though he was working long hours at his current job, he was so enticed by this interview challenge that, even after a long and difficult day thinking about a (programming) problem, he couldn't keep it off of his mind on his train ride home.
He is saying that even though he was working long hours at his current job, he was so enticed by this interview challenge that, even after a long and difficult day thinking about a (programming) problem, he couldn't keep it off of his mind on his train ride home.