This poem has a more rigid structure than many I write, but has many of the same constructs.
The poem has symmetry - the number of lines in each stanza are 1,1,4,4,4,4,4,1,1. This is important not just for the form it gives the poem, but also it reflects the construction of a bridge, often symmetrical with small spacing between the supports at the ends and larger spans "across the gulf".
Each stanza exists to move the story on one step at a time. The first invotes the reader in, the second introduces the construction of a newspaper image, made of dots. The third stanza has the space to develop the narrative of poem, instead of just introducing ideas, and begins to link the dots into shapes which make up the face in the fourth stanza. The fifth stanza moves from the process of building up the picture to printing it, mixing the printing presses and the inks with the locomotive and the smoke. The sixth does not describe the actual event itself, but the reporting of the event and the seventh introduces the transitory nature of all things, events, media reports, reputation. The seventh describes the effect on Bouch and the eighth the ultimate conclusion.
Some might believe the poem was constructed like this and then the words fitted to the construction. That is not the case. The idea behind the poem was the driving force, not the shape, although the basic four line stanza was chosen to give the poem some form that would serve as a skeleton for polishing against. A poem is not like concrete poured into a mould, but more like clay which can be moulded into a basic shape and then modified to create a final form. Perhaps the poem never reaches a final form, but eventually gets displayed as a finished but perhaps not complete object. This poem needed the short lines to give more structure. This was not difficult for the first two lines and for a long time the poem only had two short lines at the start, but in the final polish the ast two lines were split from a rather overladen last stanza and given significance of their own. The adornments are partially ornamental, to add a weight of knowledge to the poem, but also add extra important information in a cryptic form. Why cryptic? Perhaps the media is never straight forward, and although the story might seem to be told, sometimes it requires more research before the full story, if there is such a thing, comes out.
It did not take long to shape this poem. I had known about the Tay Bridge collapse for thirty years or more, was familiar with McGonagall's poem for as long and so had probably thought about the poem for some time before writing it, but not consciously. But the poem, once written on paper took surprisingly little extra work as it tended to demand this structure. And when demands are made, the only difficulty is meeting them, rather than having to design them first.
To summarise, the poem has a defined structure in the number of lines in each stanza to create a symmerty that reflects the objects and events being described, it has a narrative that is progressed one stanza at a time and it has details given in a form that reflects the implicit subject matter to enhance the narrative and educate the reader.