I'm after the name and author of a sci-fi book I would have read in the 70s or 80s, though that's not to say that the book may have dated to the 60s. The elements of the story I remember are: guy ends up on a very long flight away from earth. I think the vessel uses a solar sail and eventually accumulates speed of a significant fraction of light speed. Eventually returns to Earth where something like 30,000 years has elapsed. I recall two things as he explores a (I think deserted?) earth: an intercontinental underground rail network; his search for a cure/device that will undo the effects of his aging. Anyone able to help?
The Children of the State, which is somehow related to A World Out of Time (more on that later) has these elements from your question:
- The main character Corbell returns to a mostly depopulated Earth in the distant future (millions of years, not thousands, but he does meet someone who comes from the period in between).
- There's an underground rail network, and figuring out how to operate it (and the related short-distance teleportation booths) is a major part of the plot.
- He searches what's left of civilization for a life-extending technology which was invented while he was gone.
The Wikipedia article (A World Out of Time) says that "Children of the State" is the "main part of the novel". And another part is called "Rammer".
Since different pieces of the novel have been published separately, it makes sense that not everyone remembers all the same elements.
Other things mentioned in the Wikipedia article that I can confirm are in "The Children of the State":
- The rearranged solar system
- The woman (Mirelly-Lyra Zeelashisthar) from the past (but not as far in the past as Corbell) who tells him about the immortality treatment, and threatens to kill him if he doesn't find it for her
- The Boys and Girls, who have an alternative immorality treatment that doesn't work on adults. The Girls are gone by the time Corbell returns, and the Boys live in Antarctica.
The Galaxy magazine issues that I'm using for reference are available here: