What does Thoreau mean when he writes "we do not ride on the railroad; it rides upon us?"

he is simply using a paradox to symbolize that technology controls society.

The correct answer to this open question is the following.

What Thoreau means when he writes "we do not ride on the railroad; it rides upon us" is that we are becoming more dependent on technology.

It is great that technology has changed every aspect of our lives if we use it with moderation and for work or improving our life conditions, That is great. But there is also a negative side to it. It is absorbing most of the time in people's lives with simple and evasive entertainment that does not give people something substantial. And at the same time, people are losing personal touch with family and friends. They are not having quality time with their loved ones.