Drawbridges over waterways can carry one or two tracks. They are normally in a open, raised state to allow boat traffic to pass through and are lowered only when trains need to cross them.
In CTC, the drawbridges are controlled by routing. 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). This is similar to locking/unlocking switches, however, it takes much more time to raise or lower a bridge. Furthermore, 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.
Raised bridges will show a gap in the tracks, showing a symbol depicting a view of the bridge as it can be seen from the waterway. If lowered, this gap will be closed. When raising or lowering, the symbol will show a bridge somewhere between open and closed, with the symbol flashing. The symbol will also be flashing during the advance warning time, though with a lower frequency.
CTC maintains a reservation counter per bridge, which tells the system for how many more trains you want to keep the bridge down. You can manipulate this counter by clicking the right mouse button over a drawbridge symbol – a popup menu there allows you to increment or decrement this counter. The counter is decremented automatically when a new route is created over the bridge, thus taking 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 can you cannot keep routes continuously active over the bridge, the counter allows you to keep the bridge down during this time – otherwise you have to wait for the bridge to open and to lower again.