In these times of uncertainty a bit of escapism seems in order and so the sensible person turns off the idiot box, stops reading all those blog posts about %politician% or %black hole formerly blieved to be a solid bank% etc. and instead goes looking for some fiction. This month the sensible person should turn to Baen (actually they should do that every month) and in particular to a book by Dave Freer and Eric Flint - Slow Train to Arcturus.
Slow Train is good hard SF, low in unobtanium and handwavium but high in (romantic) comedy and otherwise perfect for a healthy reading diet. It is also low in cliches and contains original scientific thoughts, sympathetic characters and ends in a suitable fashion for sequels to be possible but not required. All in all it's good stuff that works on multiple levels from your basic escapism comfort reading to thought-provoking ideas about space colonization.
OK so enough with the generalities, what's it about? and how can you build a railway that's light years long?
The answer is simple. In some future time, after mankind has figured out how to do the living in space thing and maybe the colonize Mars thing, they have a problem. The problem is that there are lots of square, triangular and other shaped pegs that don't fit in the nice round holes that are provided for them by a caring government. They also have another problem which is that they want to find out about other star systems but realize that since FTL isn't happening the only way to get there is slowly and they have a third problem, which is that they want to build some massive 'gauss-rings' to move stuff around the solar system.
Put all this together and you have funding for a project which caapults a series of large space habitats on an interstellar voyage fo discovery with each habitat populated by a separate group of misfits. The misfits aren't for the most part trusted with the knowledge about how to run the system, they are just passengers and as the train nears each star one habitat is dropped off to live there on its own with the tools etc. to build additional space habitats out of any suitable asteroids that may happen to be lying around. Meanwhile the main computer in the front habitat sends back observations to the solar system on the activities of the passengers and of the systems they are passing through. That's the background. Now add an alien race that detects this thing coming towards them and a bunch of habitats where the original purposae has been forgotten after a few centuries of travel and you've got plenty of options for plots.
There are two bits of handwavium that I note here (and I may have misunderstood the science so perhaps they don't actually require handwavium). The first is that the slow train never seems to change direction. It seems to me that no matter what initial vector is chosen a craft will hit star systems very infrequently unless it chnages course a bit to go from one star system to the next. The second is that I find it hard to believe that a system could be built that would actually last for centuries while travelling at a significant fraction of the speed of light. However in order for our protagonists to have a story we need the train to travel from system to system and to remain in working order so we'll ignore these points,
Beyond these two points the hard SF in the book is good, indeed aside from these niggles, this novel meets most of Charlie Stross's requirements for realistic space colonization. The other problem with all space colonization books is that they have to skip over the first 100 km problem - that is to say how we get the human race out of the terrestrial gravity well - and this book does the same as many others and puts that challenge safely in the past from the point of view of the book. However to counter that problem we see a number of rules about how to build interstellar colonization trips in the absence of FTL.
Don't slow down - acceleration implies force implies energy expenditure, so don't do it unless you have to
When you do use solar sails and gravity wells
Build your habitat like a ball of (hollow) string so that you get maximum surface area in a volume.
Item 3 above is I think brand spanking new and deserves to be known as the Free/Flint law of space habitat design.
Beyond the science we have, as noted above, some interestingly different aliens one of whom becomes the primary point of view character. His observation of the various mixed nuts of humanity as he seeks his McGuffin allow us to see humans from the outside in a very sympathetic fashion. It also allows us the reader to be infodumped in a perfectly reasonable way since an alien visitor is going to have to figure things out too.
As for the plot, it starts as all good stories do in media res with the aliens approaching the "train" and manages to avoid tedious flashbacks while still filling us in on critical details about the alien physiology, the slowtrain design and so on. The plot starts fast and for the most part continues fast and furious. I'd have to go back and count but I think elapsed time for all save the Epologue is no more than a week or two. In that time the hero gets to see six habitats from low tech primitive to very high tech. I have no intention of leaving spoilers so I'm not going to explain why our hero goes on his quest. However I'll note that while this is in many ways a classic McGuffin plot the folks involved in the quest don't see it that way because they're too involved in trying to survive (and get laid), which is exactly as it should be.
All in all the verdict is go forth and purchase, You won't regret it.