Q: How do I place bridges?
A: You need to place bridges a long a stretch of already existing straight waterway, with no other entities colluding with the bridge.
Furthermore the bridge placement follows the logic of tain-station placement, which depends on the exisiting signals along the track. if you're unable to place the bridge with a certain direction, you might need to place additional signals, making the track a "2-directional" track (at least while building the bridge, the additional signals can be removed afterwards)