Drawbridges over waterways can carry one or two tracks. There are two types of bridges: normally open (raised state to allow boat traffic to pass through) and normally closed (to allow trains to cross over). They are raised or lowered depending on demand. In CTC, there is no difference whether a bridge opens vertically or horizontally. Additionally, whether a bridge is a dual bridge carrying both railroad and automobile traffic is not relevant here.
In CTC, drawbridges are controlled by routing. For normally open bridges, if a route over a drawbridge is created, the bridge will be lowered. If the route is canceled, either manually or by the train, the bridge will be raised (on 2-track bridges, both tracks need to be clear of routes). For normally closed bridges, the bridge will be raised only on demand and lowered as soon as the demand is over. This can be simulated by the foreign train feature of CTC – the “track” crosses the path of the trains above, thus allowing route interactions between the paths of the trains and the path of the ships. Note that the foreign track for the ship can be completely invisible on the scene.
Raising and lowering drawbridges works similarly to locking/unlocking switches, but it takes much more time. Before a bridge can be lowered, ample advance warning must be given to boat captains about your intention. CTC takes care of that for you, but you should be aware of it.
Railroad tracks over drawbridges typically have derail switches that divert trains off the track in case of an emergency or if a train approaches a bridge that is not in the proper position for train traffic. CTC has special mechanisms in place to ensure that the turning of the derail switches and the operation of the bridge are in the proper sequence.
Raised bridges will show a gap in the tracks, depicting a view of the bridge as seen from the waterway, with a small dot indicating where a boat would wait when the bridge is closed. If the bridge is lowered, this gap will be closed. When raising or lowering, the symbol will show the bridge somewhere between open and closed, with the symbol flashing. The symbol will also flash during the advance warning time, but at a lower frequency.
CTC maintains a reservation counter per bridge, which indicates how many more trains you want to keep the bridge down for. You can manipulate this counter by right-clicking on the drawbridge symbol. A pop-up menu will allow you to increment or decrement this counter. The counter is decremented automatically when a new route is created over the bridge, thus using up one reservation count.
This reservation counter can be used to start the closing sequence early before setting the route over the bridge itself. If you plan to send two trains over a bridge in a short time frame and you cannot keep routes continuously active over the bridge, the counter allows you to keep the bridge down during this time. Otherwise, you would have to wait for the bridge to open and lower again.