Could this be why no technology beyond steam is acceptable for Magical folk (cf. Harry Potter)? Voldemort produces EMP?

Semi-seriously, a defensive response to the threat might be to increase local power supplies. Maybe it's not so great to have the power plant in somebody else's back yard. Yes, a well-planned EMP attack with multiple blasts would probably knock out virtually everything. But maybe not if it's a single blast - US is a big country and some areas might be below the horizon. So an interesting plan might be to REDUCE interconnectedness of power systems. That way losing one region or even (say) the whole Midwest might be recoverable for areas relatively remote from there if they have adequate local power sources and communications. Duplicate your server farms in widely separate locations. Use optical communications as much as possible (less damage by EMP than wire lines). Agree, though, that even a single- burst attack (or a really big geomagnetic storm) would likely destroy the U.S. as an effective world power or even a single country even if much of it survives.

Railroad thoughts: try diesel-hydraulic locomotives again? If you need EFI for emission control, perhaps it could be retrofitted into mechanical pumps so there's a fallback? Call it a limp-home mode to get around the EPAs of the world? Makes no sense given that a diesel really doesn't need power to run for a single computer glitch to kill the whole thing (looking at you, GE and Amtrak, though EMD isn't innocent). And of course the ultimate fallback for railroaders is to requisition all those preserved steamers to move a little emergency freight around - maybe restore a few more on contingency?