In science, critical mass identifies the point where the presence of sufficient materials or energy makes a process self-sustaining.

Rail transit system starts of every kind are underway from coast to coast and North to South. Add to that most of the systems 12 or more years old are expanding their network or at least adding rolling stock to increase the frequency and passenger capacity of services.

Rail transit is no longer simply the domain of a small number of advocates crying in the wilderness. Cities and their citizens are now demanding rail transit — even if the funds are not always there to build and sustain a system.

Finding Examples

One of the initial difficulties in selling the concept of rail transit to many metropolitan areas in the mid- to late-20th century was that so few Americans had any kind of contact with rail transit. Yes, some travelers had visited cities in Europe or elsewhere in the world that had a diverse and highly integrated transit network that included a variety of rail operations, but many administrators, from the local to federal level were not quite sure how some of these concepts would translate to the U.S. environment. Want to show your city council members or county commissioners a successful rail transit operation? Where would you take them without spending huge amounts of money?

However, slowly a growing number of cities and regions managed to act on the courage of their convictions or the urgings of transit advocates and implemented rail.

In 1981 San Diego led the way by opening its first line from the city center to San Ysidro at the Mexican border with a new generation of light rail vehicles.

By the end of the 20th century, all the major metropolitan areas of California, home of the American automobile culture, embraced a variety of rail transit options. California was now seen as an example of what could be done with rail transit.