The u.i.o. I thought might be a "tie plate" for a wooden railway I thought at 
first, or a "foot" to keep rail in place on a "sleeper" or railway tie. So I looked 
around and found this which might describe it:

The part of the plate under the rail base is tapered, setting the cant of the 
rail, an inward rotation from the vertical. The usual slope is one in forty ( 1.4 
degrees ). The top surface of the plate has one or two shoulders that fit 
against the edges of the base of the rail. The double-shoulder type is currently 
used. Older single-shoulder types were adaptable for various rail widths, with 
the single shoulder positioned on the outside (field side) of the rails. Most 
plates are slightly wider on the field side, without which the plates tend to cut 
more into the outsides of the tie, reducing cant angle.

Tie plates came into use around the year 1900, before which time flanged T rail 
was spiked directly to the ties.

See also

Fishplate

http://www.absoluteastronomy.com/topics/Tie_plate