Jason, that item looks like the right size for a steam locomotive headlight window, or front glass piece.
From the photo it appears flat (front and rear surfaces parallel and flat).
The term "Lens" usually means a curved optical piece that magnifys an image. In most headlight applications (locomotives, cars, planes, ships, etc.) a curved reflector (mirror) is placed behind the actual light source and focuses the light into a beam.
The window acts to protect the light bulb and reflector from stones, rain, insects, etc. They are usually fairly thick (~ 1/2 to 1 inch) to make up for the fragile nature of the glass material. Automotive headlight windows are thin now simply because the plastic materials are so tough when compared to glass.
I am speaking as an optical lens designer, the old steam railroaders may have used the generic term "lens" for that item, we would call it a window.
It could be from a steam or diesel locomotive headlight, or a searchlight on a ship, etc. There is probably no way to know for sure. There are many applications that use similar sizes, many exterior lighting fixtures had thick glass just so folks couldn't knock out a light with a well thrown stone. It looks a little thick to be a window for a building, unless the building was in a really bad neighborhood Ha Ha.
Cheers, Kevin.
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