What does this quote by Thoreau mean: "We do not ride on the railroad; it rides upon us?"

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.