Those "nut/thread thingies" with the round heads are called carriage bolts.
They have a smooth rounded head, with a square boss just below the head on
the shaft. The boss prutrudes into the item being fastened and keeps the
bolt from turning while the nut is tightened. The rounded head is typically
very flat so it does not protrude and snag on cargo. I beleive they were
also use extensively in stockcar sides, so no bolts protruded inside to harm
livestock.

Doug Harding
www.iowacentralrr.org

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