Go with the Iron Bridge detailing! Mr. Telford's work is certainly inspiring, and I think that it's important to build some of ourselves into these fictional layouts.
Why not come up with a storyline that supports what you build. It's possible that a bridge of similar detailing could have been designed by an apprentice of Telford. Perhaps this apprentice was of Spanish decent and happened to be the son of an iron smith. The father wanted more for his son, so he sent him off to England to apprentice for Telford. The son becomes a sucessful engineer in England and returns to his home years later to build such a bridge at the entry to his village as a monument to his father. Just and idea, but I do think that liberties can be taken to suit our own tastes.
As for the tree, it's perfect: it would be a crime not to use it! I think you could make some very realistic foliage for it by replacing the needles with Woodland Scenics Fine Leaf Foliage
. There are plenty of on-line hobby shops that sell it, so I don't think that it would be too difficult for you to get.
Nice work, keep the updates coming!