“Was that really the primary purpose of the interstate highway system, or did they just say that in order to justify spending Federal money on it?”
No, it was really the purpose. Eisenhower was faced with a choice of developing rail transport or creating the interstate system. Based on his observations during WWII, he chose the autobahn.
It’s much easier and quicker to bring a stretch of bombed-out road back into service than it is a stretch of bombed-out rail.
The first modern U.S. highway system came into existence around World War I. These are the roads you now see designated as the old Federal routes US-1, US-66, US-22, etc.
Interestingly, part of the political pressure for a national highway system in the early years of the 20th century was a general feeling in this country that the railroad industry had simply become too powerful. Most of the companies in the original Dow Jones Industrial Index were railroads, and the decades after the establishment of the railroad industry in the U.S. saw frequent widespread economic chaos due to railroad strikes and other service disruptions.
I’ll note here that passenger rail largely died out with the major implementation of the Interstates in the 60’s.
I’m totally OK with that. Just sayin’.
The Eisenhower Interstate Highway system was also designed to provide about a one mile long straight stretch every so many miles, so those could be used as runways and landing areas for military aircraft.