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.

Thoreau refers to the railroad several times in Walden. It is a symbol of progress, haste, mechanization and all the things he is rejecting by living in deliberate solitude in the woods. Thoreau asks a series of questions about the railroad. Where do we expect them to get us? Why do we want to rush frenetically around the country when there is so much to see and experience at home? Finally, he says:

We do not ride on the railroad; it rides upon us. Did you ever think what those sleepers are that underlie the railroad? Each one is a man, an Irish-man, 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.

This is a startling image. People have labored and died for the railroad. Like the Great Wall of China, it is built over the bones of those who worked to build it. If we feel any common humanity with them, then the railroad “rides upon us.” The bitterness of this thought is emphasized by the unseemliness of the double pun in “sound sleepers,” meaning both the sturdy railways sleepers and those who have died in the service of the railway soundly sleeping the sleep of death.

The railroad rides us in another way too. One of Thoreau’s major themes in Walden is the extent to which technology, which promises to free us by making everything faster and easier, actually enslaves us. Such improvements, says Thoreau, are at best “external and superficial,” a pointless distraction from the simple life he values.

We have to pay to ride on the railroad. In order to be able to pay, we have to work to earn the money. So although we seem to be traveling in fast and without effort we do not take into consideration the amount of labor we had to put in to pay for the train ticket. In Thoreau's opinion it would be better to walk to wherever we are going, and probably better still not to go very far at all. His friend Emerson said, "Travel is a fool's paradise." Thoreau wrote: "I have traveled quite extensively, in Concord." He didn't believe you had to travel very far to see interesting and beautiful sights. Some people travel great distances without really seeing much of anything. The railroad just adds complications, expenses, and stress to life. The French poet Charles Baudelaire wrote a cynical poem about travel titled "Le Voyage," in which he includes this exquisite line:

Amer savoir, celui qu'or tire du voyage!

Which can be translated as: What bitter knowledge one gets from traveling! Distance lends enchantment. We think that far-away places are going to be special, but when we get there we find that they are often very ordinary. As Emerson says, "Our ghost goes with us." They lose their glamor just because we are there. We can't escape from ourselves. 

Most of American life consists of driving somewhere and then returning home, wondering why the hell you went.
                                                                     John Updike

He had followed the parkway as far as New York, and all the way, there had been a constant stream of cars, two and sometimes three lanes of them in both directions--a movement so implacable it looked like a headlong flight. Their brows furrowed, their muscles tensed, the drivers, often with whole families in the back seats, charged straight ahead as if their lives were in jeopardy, some of them not knowing where they were heading, or heading nowhere in particular, just desperately filling the empty hours with noise and speed.
                                Georges Simenon, The Rules of the Game

This is an example of paradox: The statement can't be true, but it is true. When taken literally, it is obvious that railroads do not ride upon their passengers; trains ride upon tracks. However, when interpreted figuratively, there is truth in the statement--Thoreau's truth. He means that we do not control our technology, that it serves instead to control us. In Thoreau's day, the railroad represented modern technology as rail lines were expanded to cross the continent. He viewed this "progress" as being, in fact, a negative--one that acted to destroy the natural world, pollute the environment, and reduce the quality of life by making it more complicated and fast-paced.