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Larry, fish-plates were the plates they used to use to bolt railroads together. The name was originally a naval term for the braces on a timber mast. Similar plates were used to join lengths of timber in construction, with wooden pegs, bolts or even nails. Modern fishplates have multiple spikes, pressed into the 2 parts to be joined, usually on both sides.
Here's a modern type.
http://www.biosdobris.cz/obr/stycnik.jpg

I always heard them called flitch plates. Might be a regoinal thing.

http://en.wikipedia.org/wiki/Flitch_beam

So many ways in which American English is slightly different from my English. We say railway, you say railroad, we say sleeper, you say tie. Fish came from fiche [as in microfiche]- a plate, and not from haddock and the like! To me, a flitch is a dod of wood squared up for re-sawing or an uncut side of bacon.

http://en.wikipedia.org/wiki/Fishplate shows the plates in their many guises.