Remnants of Pine Lake Bridge
https://www.estately.com/listings/info/22-lima-grade-rd--3
BH Photo #479950
This is your mystery bridge Jim, or what's left of it. Unfortunately, Historic Aerials imagery only goes back to 1984, and the bridge was already gone. Melissa found it mentioned on Pinterest about it collapsing, but nothing more so far. So if you indeed crossed it in 1978, then it was gone within the next 6 years.
was this a part of the Grade/Cloquet RR.