Our steel industry declined for one big reason: the three major waves of steel consumption ended — railroads (mostly built out by the 1920s), highway bridges (during the construction of the Interstate Highway System in the 1950s and 1960s) and automobiles (from the 1950s until the 1980s when much of the steel was displaced by lightweight plastics and composite materials in cars).
“In 1945 the U.S. was the only major country in the world whose infrastructure and industrial capacity was unscathed by World War II.”
And the government and unions thought the good times would never end. Surprise!
Not quite accurate, there's always been a market for railway cars, the Budd Company (then spun off to become Transit America) made railway cars up until 1987 when they closed their doors after losing a major contract to your Bombardier which was allegedly subsidized by the Quebec government. Budd subsequently sold the division to Bombardier.
Bombardier then sold its train making division to French train giant Alstom SA back in 2020.