Travel can induce such a distinct and nameless feeling of strangeness and disconnection in me that I feel insubstantial, like a puff of smoke, merely a ghost, a creepy revenant from the underworld, unobserved and watchful among real people, wandering, listening while remaining unseen. Being invisible—the usual condition of the older traveler—is much more useful than being obvious. You see more, you are not interrupted, you are ignored.
Ghosts have all the time in the world, another pleasure of long-distance aimlessness—traveling at half speed on slow trains and procrastinating. And this ghostliness, I was to find, was also an effect of the journey I had chosen, returning to places I had known many years ago. It is almost impossible to return to an early scene in your traveling life and not feel like a specter. And many places I saw were themselves sad and spectral, others big and hectic, while I was the haunting presence, the eavesdropping shadow on the ghost train.
Paul Theroux, Ghost Train to the Eastern Star, 5-6