A major factor in supporting expansion of intercity passenger rail from what the situation was in 1971 is the growth of city rail transit systems, be it heavy rail metro, commuter & regional rail, light rail, street cars. In the 1950s and 60s, many cities shut down their street car and remaining interurban trolley systems. In 1971 at A-day, the only cities that had surviving extensive rail transit systems that come to mind are NYC, Philly, Boston, Chicago with smaller systems in Cleveland, DC with several commuter rail lines, Buffalo?, Pittsburgh?
Also, if you don't restrict trains to just Amtrak trains, but look at the bigger picture with commuter rail and urban rail, there is considerably more mileage being operated today than in the 1970s and considerably more ridership, and the trend is clearly continuing with several more projects either being built or likely to be built soon. That means more people are likely to either use rail service (of some sort) more or less regularly or even if they don't use it themselves, they may work with people who do or have family members or friends who do so the whole concept of passenger rail is becoming more and more normal and embedded in people's awareness. That means that whereas people once thought of the car as the first and basically only way of getting anywhere, with there having to be a very compelling reason to even consider any alternative, people are becoming more open minded.
Since then, Washington DC has undergone a significant transformation with the DC Metro system along with MARC and VRE commuter rail. Baltimore has a single metro line and a light rail line. Atlanta has MARTA. Miami has a single metro line and Tri-Rail. LA now has subway and light rail lines and has plans for an extensive transit system. San Francisco and the Bay area have BART, Muni Metro, streetcars, etc. A number of cities now have light rail systems which they did not have in the 1970s.
All the cities with local rail transit systems provide an improved foundation for intercity passenger rail service to connect the cities. Part of it is yes, increased familiarity with the concept of taking a train for transportation as opposed to the 1950s and 60s when the automobile & suburban sprawl were in ascendance. But another part is more cities that people can travel to by train where they don't need a car to get around the core part of the city region or have to to spend hours figuring out the local bus system.
The growth of local rail transit systems will in the short and long term help drive the development, or depending on your point of view, restoration of corridor trains and in turn provide a better foundation for the LD trains.
Electronic gadgets and the internet - laptops, tablets, smart phones - are also a major factor as people can't text, check emails, or get work done while driving (well, we hope so). Not easy to use the gadgets on an airplane either with having to shut down below 10K feet or having to hold the smartphone 5" away from your face when the person in front of you puts the seat back. On the train, text away or watch movies on your laptop before the train even leaves the station.