This sentence from Walden occurs in the context of criticizing technology for making our lives more complicated. Thoreau says we, as a society, believe we must have more railroad tracks so that we can travel to ever more distant places, but then asks, what is the point of all this travel? Aren't we better off to be content with the simplicity of staying at home?

But this is also a pointed statement about social justice. He is saying, quite literally, that the railroads ride on the corpses of the men who died in accidents while they were being built. In the same passage he states:

Did you ever think what those sleepers are that underlie the railroad? Each one is a man, an Irishman, or a Yankee man. The rails are laid on them, and they are covered with sand, and the cars run smoothly over them. They are sound sleepers, I assure you.

A "sleeper" is both a term meaning a railroad tie and a euphemism for a dead man, "asleep" in the grave, so Thoreau is using the word as a double entendre. He is saying that railroads are not an innocent technology that simply springs up from the earth. Instead, he argues, every length of railroad we build costs human lives.

Thoreau also implies that railroads "ride upon us" in becoming another place of excess that robs us of the simplicity of seeing life as it really it is:

I think that in the railroad car we are inclined to spend more on luxury than on safety and convenience, and it threatens without attaining these to become no better than a modern drawing-room.

This quote comes from "Walden" by Henry David Thoreau.  In it, he is arguing that we have come to be too materialistic (which is, of course, a large part of why he went to live by Walden Pond in the first place).

What he is pointing out in this quote is that we make things, like railroads, and we think that we are using them to our advantage.  But in reality, he says, we are slaves to them once we have them.  Instead of living life, we are always trying to acquire more things.

You might think about who is using whom in the context of cell phones today.  They started out as a convenience, but now they make it so a lot of people can never relax because they're always available for people to call them.