I'm not sure it's a replica, as I don't know why they would bother to add a fictitious maintenance (?) date.
It definitely looks narrow gauge to me, as it seems to be sitting quite low to the ground.
I checked for railroads around Hinckley, MN and there are some SG lines. However, there were a good number of NG logging lines in the region.
I can't decide if that's O.A.R.K. or G.A.R.K. In any event, I didn't get any hits for it in MN.
Unfortunately I wasn't there on an operating day but I measured the operational trackage in the museum grounds and it was 3 foot gauge, I thought it was a curious choice for a Japanese operation but their operating loco came from South America I believe. I've since found a better photo of the train with caboose.
There is a similar looking Rio Grande NG caboose with the same number but it is on the C&TS. My thought was that it was a replica and O.A.R.K or G.A.R.K. was the builder but like you I couldn't find any reference to them anywhere.
Just one of the many mysteries of Japan, you never know what you will find behind a door or around the next corner.
Edited by westfalen, 11 January 2017 - 10:27 PM.