Not sure how a historian would describe this phenomenon, so please forgive the vague title. It's something like a social philosophy where most people have a single, shared vision of what the future should be, and this vision is implemented by a powerful, centralized management and planning authority. Some examples might be:
In the 50s, Robert Moses tore down entire New York City neighborhoods to build huge freeways, according to his master plans. A few decades later, urban infrastructure projects on that scale essentially disappeared.
There was a project called "Operation Plowshare", to use nuclear explosions for large-scale civil engineering. It was terminated in 1977.
The "Reber Plan" called for building two large dams, among other infrastructure, across San Francisco Bay. Since the 70s, these kinds of projects would be blocked on environmental grounds.
The American space program during the 60s was driven by a single, top-down goal of reaching the Moon. Once that happened, some people thought it would be followed by commercial space travel, like how the 50s saw commercial jet travel. But during the 70s and 80s this sort of fizzled out, with the Space Shuttle program doing ever-fewer launches at ever-higher cost compared to original visions.
Not technically American, but the Concorde was designed by the French and British governments during the 50s and 60s. The previous decades had seen a quick succession of ever-larger, ever-faster planes. But after Concorde was introduced in 1976, there wasn't really any successor. The supersonic jet program just sort of stopped.
In 1954, the chairman of the Atomic Energy Commission famously said "Our children will enjoy in their homes electrical energy too cheap to meter", from nuclear power. Of course, that didn't happen. The last US nuclear power plant started construction in 1977, according to a quick Google.
After decades of vaccination campaigns, smallpox was eradicated in 1977. Since then, no other human disease has been similarly eradicated (although as of 2016, it looks like polio might be getting close).
Airline routes, fares, and schedules used to be centrally planned under the Civil Aeronautics Board, until the Airline Deregulation Act in 1978. Likewise with railroads and the Interstate Commerce Commission.
A great deal of scientific and engineering work was done at the famous Bell Labs, which was technically a private company but had a government-like monopoly on phones. Bell Labs still exists, but this type of research there tailed off a lot after Ma Bell's breakup in 1984.
Of course, one could present ad-hoc explanations for all of these - Robert Moses fell from power because of Nelson Rockefeller, supersonic jets didn't work well because the sonic booms were too loud, and so on. But it does seem like there might be some kind of underlying cause, some kind of "social mindset" that most powerful people adopted and that was eventually thrown out/faded away. How would a historian describe this phenomenon, and why was it so much bigger in the 50s and 60s than it was in succeeding decades?
So, I am not going to really comment on Thiel outside of saying that doing so would violate the 20-year-rule and probably the rule against incivility. Also, I would wager that the "most people" that shared such a vision did not include those economically and socially marginalized for several reasons...
Anyways, Tony Judt describes this phenomenon as in large parts being a legacy of World War II. During the war, the full power of the modern state - and especially the modern European state - was mobilized for the first time to a rather full extent. During the war with the goal of conquering people or defeating an enemy. Great Britain for example spend half of its Gross Domestic Product on the War Effort in 1945. And then came the post-war experience of managing hundreds of thousands of refugees, Jewish, Soviet, German and otherwise; the population transfers in Europe; the rebuilding of a destroyed urban and other landscape; and re-taking the industry that the Nazis had de-facto put in the service of the German war effort.
From all these experiences and from the way they were tackled, e.g. UNRRA and the refugees, sprang what Judt terms the "political religion of post-war Europe": the faith in planning. The experience of the inter-war period, the problems immediately after 1918, the stock market crahs of 1929, unemployment, the inadequacies of inter-war democracy and capitalism that had tempted people into authoriatarianism; this all seemed to the people rebuilding at that time - who had large experiences with managing a war economy that by its nature must be planned - connected by the failure to organize society better. Society should be organized and planned better with the welfare of the people in mind. This of course didn't mean planning in the style of a planned economy a la Soviet Union, though that too had a certain appeal since the USSR had weathered the crisis of capitalism of 1929 and had withstood the Nazi onslaught in large parts due its planned economy.
Planning in this instance was more meant as the ability and duty of the government to solve large-scale problems by mobilizing and directing people and resources to collectively useful ends. This stemmed of course in large parts from national governments being virtually the only institutions in post-war Europe who had the means and resources to tackle such huge tasks like rebuilding country and economy. Plus, most people saw what had been possible in the war if a national government decided to fully mobilize all the resources it had at had. A very pervasive view was that if a government could with the help of planning and control defeat the Nazi armies, it also could solve problems such as poverty, unemployment and homelessness.
What virtually all of these planners had in common was the idea of an enhanced role of the state in social and economic affairs. And this made a lot of sense at the time. In Britain for example, while little actually planning took place, the government had after the war already held control over such crucial industries like mining and transportation. The idea that the same techniques that had conquered Germany could also be put in service of the people and society made sense to those who had organized the previous effort and gave rise to the idea of a nationalized health care system in Britain.
Of course different countries took different approaches (France e.g. was very heavily into actual planning) but one other thing they all shared and that was also practiced heavily in the US was public investment as part of this campaign to use the awesome resources of the state in the service of economy and society. The idea was that in the recovery after war must prevent at all costs a return to the economic stagnation, depression, and unemployment of the inter-war years. This not only gave rise to the European style well-fare state but also to many of the projects you mention above.
And in many ways, this policy during the 50s and 60s also showed success, giving it more credence and a bigger cultural impact. West-Germany could chalk its "economic miracle" of the Adenauer and post-Adenauer years up to it. The US and within it the military-industrial complex, which also rose from this mindset, could claim military superiority over the Soviets, putting a man on the moon and increasing the living standards for hundreds of thousands of US citizens through such programs like the GI Bill.
The reason, why this kind of approach by the 70s started to fail and ultimately fade are varied and range from economic factors such as the oil crisis to cultural factors to the workings of hegemony within capitalism but the historic roots for this phenomenon can be found within the bad experiences of the inter-war years and the success of planning and governmental involvement in the war effort.
(Note: Not sure why the "travel" header was added to the title. Transportation projects were a big part of this, of course, but it's not really a "travel" question.)
It wasn't the era of optimism, it was the era of super bureaucrats.
Prior to the 1930s, the US federal state was quite minimal. FDR and WW2 changed that, for good or ill, but the way they did it was extremely peculiar. Because of the way FDR operated, and because of the extended crisis that WW2 caused, the US didn't just found a bunch of bureaucracies in the period in question (roughly 1930-55) but they founded bureaucracies and then left the same people in charge of them for a very long time.
In most bureaucracies, the amount of power the person at the top has, however theoretically unlimited, is quite limited in practice because of immense webs of institutional resistance. CEOs can theoretically fire everyone and replace them, but they never do, because it would generate as many problems as it solves. The super bureaucrats got around this problem because they had the luxury of building their organizations from the ground up. Hyman Rickover, J Edgar Hoover, Robert Moses, Curtis Lemay, all were pushed into immense power at unusually young ages, then managed to hold onto it long after the crises that pushed them there were long past. They didn't have to deal internal webs of resistance because they personally hired everyone in their respective organizations. Their organizations were theirs, through and through, in a way no boss of any mature organization can dream about.
Because everyone in these organizations was hand selected, they were all on the same page, which meant that they could accomplish a lot. Note, "a lot" and "a lot of good" are not the same. For every Hyman Rickover who built the world's largest fleet of nuclear reactors without ever leaking a single rad, there was a Hoover spying on whomever he felt like, constitution be damned, or a Robert Moses deliberately building freeway overpasses too lower for buses. But at the core of it, that's why the 50s and 60s were full of crazy projects, because the commanding heights were being run by people who were super empowered by the organizations they'd built, and they could do whatever they wanted to do.