"Almost exactly 75 years ago" sounds like Pearl Harbor to me - and you have confirmed the USA anyway.
I know one of the first "heritage" lines was the Edaville in New England, and Wikipedia says they got their stock from the Bridgton & Saco River RR, so I'm going to guess that.
You've got it -- certainly near enough to do the job. The Bridgton & Saco River, alias Bridgton & Harrison, was the last of the several 2ft gauge railways in the state of Maine, to keep a passenger service. The line closed completely in September 1941: a railway-enthusiast group was formed to look at ways and means of somehow reviving the line and running it largely, anyway, with volunteer labour. Then, as you say, Pearl Harbor and what resulted therefrom, came about. Frivolities such as railway preservation had to be shelved for a while: by the time life returned to normal, the railway had been dismantled, and the chance to save it "as had been", was gone. Locos and stock were still in existence, and -- as above -- were acquired for the ("purpose-built") Edaville line. Said outfit also got some motive power and stock from the 2ft gauge Monson Railroad in Maine, which outlived the Bridgton & Harrison by a little while; but was way up north in the very back of beyond, and had lost its passenger service long since.
Giving you the highball, buddy...