OP is saying the opposite: the TCR required huge investment, and was massively speculative, with a huge bubble ensuing. But rail travel did revolutionize the world, and few people sat around asking whether or not it was worth it.
https://www.amazon.com/Iron-Empires-Robber-Railroads-America...
it was hugely controversial in Congress.
I don't happen to know about roads, since that wasn't one single thing. But I doubt they happened because of blind faith in the future, either -- it was probably more businesses demanding roads NOW. So the argument that no one questioned them is dubious at best.
What I meant to say was that the AI bubble is feels different in that there seems to be relatively more skeptics versus railroad mania or the dot com bubble (but this could just be skewed by my own experiences).
Whether or not this suggests a flawed value prop for AI is another story.