Some interesting picture & video there local Hiker. The concrete sign with the numbers on looks like a bridge identification sign (all railway bridges have an infrastructure number) and would tally with an infrastructure register. According to the 1911 South Eastern & Chatham railway list of bridges, bridge 745 was underbridge 'Kingsferry Road B' of steel girder construction and located between Queenboro and Sittingbourne stations (island side) of the Swale.
I can only assume that the vent pipe you mention is for the release of a build up of Methane caused either by organic waste being buried there or perhaps for 'Marsh' gas?
During my time on the railway in the 70/80s I visited the lifting bridge at Kingsferry on several occasions (the railway signalling controls when the bridge may be lifted). I was fortunate enough to go to the top of one of the legs and also through the cable tunnel which runs underneath between the mainland and the island. Walking down the stairs in the hollow legs to the bottom to my young eyes was like entering the set for a 'Bond' villain's lair.
Around this time there was a chap who used to make steel drums working out here. This entailed getting an old steel oil drum and initially hammering out the 'hollow' in the end before tuning individual sections to give the distinctive Caribbean sound. You can appreciate that the initial hammering was quite noisy and the solitude of this area was the perfect workshop for him to carry out his noisy task. In itself this was quite skilful because the metal needs to stretch as it becomes concave and so a degree of judgment would be required as to the structural integrity of the steel.
The section of railway line on the bridge span is not electrified at 750v for practical reasons and therefore only electrical 'multiple units', locomotives running on diesel or two or more electric locomotives coupled together are allowed across the bridge to mitigate the chances of becoming electrically isolated whilst traversing the span.
In my late teens early 20's we used the night clubs on the island on Saturday nights at either Sheerness (Woody's) or my favourite at Leysdown (Stage 3), and some evenings the announcement would be put out from the DJ stand that the Kingsferry Bridge would be raised at 2AM for maintenance (usually for renewing on of the many lifting cables). This often led to an almost 'Le Mans' start from the car park(s) and a procession of cars trying to get back to the mainland before getting cut off!
I believe that the maternity unit on the island which served the Swale area was closed for this reason as well, an on occasions ambulances heading to Sheppey were held up by the raised bridge.
