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.
These national obligations to the states illustrate the concept of federalism because it is the national gov's responsibility to its separate but contributing, building blocks.
