Stew,
I know exactly what you're talking about. I'll try to explain what's happening, and maybe a way to resolve the issue.
As you are probably aware, the bridge is really a scenery object with a piece of invisible track placed over it to allow the train to drive across the bridge. To close the bridge as the train approaches, there are triggers placed on either side some distance away. The bridge in essence is there for show with the invisible track providing the real track for the train to follow.
Now to get your signals to stay red, it maybe possible to setup some invisible junctions and tracks on either side of the bridge. The default position of the bridge-junctions then would be reversed so the track circuit is open before the train approaches. This will keep the signal red. When the train approaches the bridge, the AI driver should change the junction lever to the normal position, causing the bridge to close, and the signal to turn green.
The problem I see is the AI changing only one signal then stopping to change the other once the driver crosses the bridge in typical AI driver fashion. The way around this is possibly using a junction controller to link the two junctions together so they work in tandem like a crossover would.
Hope this helps.
John
