The state of the rail system is a good proxy because it's a huge infrastructure project that doesn't generate any immediate profit. These types of projects are done primarily to provide social value, thus it's an indication of a society that is willing to invest into improving the quality of life for the majority.
That seems like a rather arbitrary measure. I could just as easily state “The state of the cereal aisle is the most reliable measure…”
The state of the rail system is a good proxy because it's a huge infrastructure project that doesn't generate any immediate profit. These types of projects are done primarily to provide social value, thus it's an indication of a society that is willing to invest into improving the quality of life for the majority.