The penultimate verse seems to refer to the viaduct carrying the line across the Tay into Perth opened in March 1849 - the year after the reported first performance of the song. If so; the song describes the view of the viaduct under construction. [i]
This detail from a picture by James Hall Cranston shows the new railway bridge soon after it was opened.

Reference: [i]   https://en.wikipedia.org/wiki/Dundee_and_Perth_Railway