Officially called THE NEW FORTH BRIDGE SONG or THE YOUNG ENGINEER, it can be heard on Folktrax FTX-225: The Ironmasters (music not online)

A few more words I found from a Mudcat thread:

"When the men from the south and the men from the north
Will meet and complete the bridge over the Forth."

According to another post, the words are included in the songbook Songscapes, but buying a copy of that book nowadays looks to be quite pricey; maybe a library near you has a copy? Check https://www.worldcat.org/title/songscapes/oclc/50464360

On another thread, 'catter "nutty" (who last posted Nov. 2019) seemed to have organized publishing efforts, and user "vectis" (who was active as of July) mentioned wanting a copy. You could try PM-ing one of them in hopes that they have a copy of the book still. (see the thread I'm referring to here).