Just before the line enters St Petrock Station it crosses Tredinnick Creek on two girder bridges. Photos of the bridge over Little Petherick Creek on the Padstow branch were studied carefully. It's a well-photographed spot, especially since it became part of the Camel Trail; here's one of my own photos. Whilst my model wasn't intended to be a copy, I wanted to get the 'feel' of the bridge - especially those metal cylinders that support the structure in mid-stream.
I started by building a couple of Dapol girder bridge kits. These were being sold by Airfix when I was a kid (which was a long time ago) and have stood the test of time quite well. However, the trusses on the top are like nothing I've ever seen and are surely structurally unsound. Perhaps they were made that way to clear the old Tri-ang overhead wire system. I replaced them with something a little more authentic from the Plastruct range.
The supporting cylinders were made from 22mm plastic tubing - somewhat undersize as the originals were 8ft in diameter (=32mm in 4mm/ft scale). I couldn't get anything suitable from my model shop, so bought a length of plastic pipe from the local DIY shop. Though doubtless perfect for plumbing, I quickly discovered that it's made from plastic that can't be glued with modellers' polystyrene cement, so I wrapped each cylinder with thin plasticard, held in place with Evo-stik. The bits between the cylinders are Plastruct again.
Photos of Little Petherick bridge in operational days show boarding down the middle of the track. There were also safety rails to keep trains on the bridge in the event of a derailment; in my version these are lengths of flat-bottomed rail, mounted upside down on C&L chairs. I think they help to give the bridge a nice chunky appearance. Finally, the old bridge had handrails to guard the workers from falling in the drink.