Probably a novel. Maybe a novella.

Ken had the unusual ability to let his mind wander and piggy back onto the lives of others for a little while, which was a nice change of pace from his office job. One day, he does this again and piggy backs onto a very unusual man who can not only hear him in his head, but it's a ride that he can't leave.

Stuck as a voice in this man's head, he is brought into a dark conspiracy involving those who can do just what both of them do and strange men on the trains every day looking for them.

Ken - A man with a rather boring job and a peculiar ability.
Other Guy - The one that he gets stuck in.

A world much like our own, but the majority of the intrigue happens on the skytrain between Columbia and Waterfront Station on the Expo line.

I was on the skytrain and thought of a weird super power. The ability to piggy back on the lives of the person whose seat you just took after they got up to leave.

I really love the twist in this one. It's great. I hope I can do it justice.