The spending planned by Alphabet Inc., Amazon.com Inc., Meta Platforms Inc. and Microsoft Corp., all in pursuit of dominance in the still-nascent market for AI tools, is a boom without a parallel this century. The search for a comparison to the spending projections — which came as the four reported earnings in the past two weeks — requires going back at least as far as the telecommunications bubble of the 1990s, and perhaps to the build-out of the US railroad networks in the 19th century, the postwar federal investments in interstate highways or New Deal-era relief programs.