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

This
sentence from 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
afor 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.