There are a number of threads on OGR about lift bridges, both manual and powered, including a current thread called "lift up bridge". There are three key features to such a bridge: First, the hinge point need to be higher than the top of the rail. The other thread shows this quite nicely and I think something you could easily incorporate into your bridge - e.g., a short aluminum plate (say 6 inches long) the goes under one end of the bridge with two 'ears' for the hinge (which would be two short threaded screws). On some of the other threads, you'll see using a pair of simple butt hinges mounted on wooden blocks, so the hinge point is above the top of the rail.
The second important feature is to have some sort of alignment pin or slot at the other end of the bridge so the track will be aligned when the bridge is lowered into place. In the current 'lift up bridge' thread, the author claims that his design is rigid enough laterally so that there is no mis- alignment when the bridge is lowered.
The third feature is to have some sort of control block on either side of the bridge and a microswitch or other type of switch that would kill power to the rail in those blocks when the bridge is raised, so you don't inadvertently drive a train into the raised bridge - or into the abyss on the other side...
Searching for 'hinged lift bridge' will bring up a good discussion and, courtesy of Susan Deats, this link: https://slsprr.net/features/bridges1drwgs.htm which shows some construction info. I don't think you need a gas-spring for what you are doing - I should think just designing it so the bridge goes a couple of degrees beyond 90 deg and a stop to rest the bridge on when up would be sufficient. If lifting the bridge is an everyday occurrence, you could use a linear actuator to actually lift the bridge up from below. These devices come in lots of sizes and power and some run on 12VDC.
