Mars is my Destination by Frank Belknap Long

There were twenty-five or thirty passengers wedged into the middle section of the train, all standing in slightly cramped postures and most of them unsmiling. I knew exactly how they felt. Not being able to get a seat in an off-hour in the evening can be irritating. But right at the moment there was no room in my mind for annoyance. A slow, hard-to-pin-down uneasiness was creeping over me again, as if a pendulum were swinging back and forth somewhere close to me, ticking out a warning in rhythm -- and I couldn't shut out the sound of it.

Just my over-strained nerves, of course. How could it have been anything else? I turned and looked at the man standing next to me. He was middle-aged, conservatively dressed, and had a square-jawed, rather handsome face, with a dusting of gray at his temples.