The paths are seperate to the models in GTASA and they do not need to move. When the bridge is swung open the tracks would not be meeting each other, so no trains could get across anyway. This is why it would require careful SCM coding to ensure the bridge never opened if a train was anywhere near it.
Ideally, this would done by creating a "no vehicle zone" like the lift bridge in GTA3 had when it started to rise.
There is a helpful page about swing bridges on Wikipedia
which shows how they work. Basically, a long section of track rotates by 90 degrees, causing a gap to be opened up either side. I have in mind is bigger (and somewhat higher) than this old swing bridge (645x778, 123kB)
mixed with a more modern swing bridge (1363x1025, 130kb)
but built so that it suits Myriad.
Having found an aerial view of those two bridges
, I think that some portions of the railway could be on a causeway, like an artificial ridge or embankment. Illspirit has expressed this idea before and now I've seen that curved bridges are sometimes replaced with this sort of thing, it looks like a good idea. It would add even more variety whilst saving a lot of polygons!
(EDIT) The swinging section would need to be big enough for JasonB's "Port Orchid" ferry to get through one side, although I don't know how big he wants that to be. The bridge will be quite high above the water so that most of the original boats in GTASA could get under it quite easily. Probably about the height of the bridges to illSUB.
(EDIT2) Illspirit seems pretty happy with the "Idea 06" route, so he made a few rough pieces of track in DFF format for me to test whether the route would be feasible in a 3D mapping tool. After spending all day shunting and tinkering with the Z axis position of the railway objects, it looks like the northern route will be feasible:
(800 x 600, 51kB
or 1600 x 1200, 200kB
You can't see the route along the East coast of illURB too well from the smallest image. The railway gradient which will be 0% over the swingbridge, then stay at 0% for a while. It will gradually start to steepen as it approaches the illURB coastline, making landfall with a gradient around 3%, which becomes 5% as it runs through cuttings and along embankments on the coastline. The first parts of the slope could be part of an artificial "spit" created as a coastal defence, which will make it less obvious than a sloping metal framework. The route enters the Black Mesa Tunnel where the ground suddenly steepens and begins to curve to the left.
It exits from the Black Mesa Tunnel to the south-west of the mountain in fairly straight cutting, becomes an embankment as you continue South. A minor railway station could be somewhere along on this section, since there is plenty of open space and it's near to the freeway. The embankment slopes gently downward at a gradient of around 2% and begins to curve towards the right. It cross the freeway on a large bridge and joins onto another embankment.
The railway continues to slope downwards at about 2% as it travels West, right the way across illURB. It levels off to a gradient of 0% once it makes the East coast of illRES, at the same level as the 4-lane road. It crosses this road via a level crossing and continues at a gradient of 0% as it travels West. It enters a cutting which gradually deepens as it travels into the wall of the Crater but the railway stays at a gradient of 0% for this. Once the cutting is deep enough, it becomes a tunnel and starts to curve left inside the crater wall. There might be a very slight gradient through this portion.
The railway exits from the South-east portion of the crater wall, heading roughly South but continuing to curve left, towards the South-east. It crosses the first two lane road via a level crossing and an embankment begins almost immediatly as the terrain falls away. A fairly small bridge is used to cross the next 2-lane road and another embankment carries it to the four lane road, curving to the left this time. A bigger bridge is used to cross the four lane road and the railway heads south towards Necrosis.
And that basically completes the Northern route!
I don't have JasonB's South-east illCOM lot or Necrosis, so I can't start laying out a decent draft of how the Southern sections of the railway will be yet.