I hesitate to appear to black cat the OP's story, but the attached link may also be of interest regarding a flight under not just the Forth Bridge but the Forth Road Bridge as well by by no fewer than four Fleet Air Arm Buccaneers under cover of darkness
https://www.heraldscotland.com/news/...-under-bridges. Of course, the Royal Navy had been there much earlier, as evidenced by the attached report of a particularly daring flight by a local boy, no less than Captain Eric "Winkle" Brown, in a Seafire - an amazing feat that was evidently blamed on the Royal Air Force!
https://www.pressreader.com/uk/the-s...82372628323512. I'm a local boy too and somehow can't help feeling that there must have been many other, unrecorded flights under the original bridge, especially considering what a tempting prospect that must have been with such a prominent landmark.
A couple of footnotes: firstly, and whilst I can understand a possible need for additional clarity now that there are three bridges over the Forth between South and North Queensferry, the rail bridge is *the* Forth Bridge and not the Forth Rail Bridge. Secondly, there was apparently a rather different problem during WWII when an American destroyer received orders to anchor one nautical mile above the Forth Bridge. On subsequently receiving an urgent message seeking clarification as to why he had not obeyed the order, and seemed to be about to ram the Kincardine Bridge, some miles further upstream, the American captain allegedly signalled back, "Your message not understood. Have only passed the first bridge"!
Jack