Page 33 for the high voltage medium sized unit facilities. Larger / higher voltage units are made by European facilities. Some U.S. facilities are considered sufficiently knowledgeable to produce large transformers; but, haven’t actually built any of the rail-car sized units to date.
Could likely operate transmission lines at a derated voltage / power capability, with the available locally sourced units.
https://energy.gov/sites/prod/files/Large%20Power%20Transformer%20Study%20-%20June%202012_0.pdf
How would they build them after a successful EMP?