> once built, railroads provided economic value right off the bat

If they were laid on a sensible route, completed on budget and time, and savvily operated. Many railroads went bust.

Equating this buildout with LLMs is also a category error. Waymo (self-driving cars) depends on the same infrastructure, and there are a variety of other robotics programs which are actually functioning, you can see them in operation. They all require a lot of GPUs to train and run the models which operate the robotics.