Thank you both for you replies.
Yes Jason I agree if the engine had been produced the sledge would have been wooden.
Most of the pictures I have seen show the workings mounted on an I beam frame and then mounted on the sledge.
I am trying to produce a model of the 1862 patent drawing "somebodies thoughts" .
I was in two minds about how to make the sledge but felt that a model of an "idea" was allowed a bit of " license" and felt that a solid frame followed the drawing and was more substantial if I wanted it to run.
I look forward to your further input as the drawing is a bit lacking in detail.
Ken