Yes, that´s possible and there is even the possibility to place (transparent) cities in water-terrain if these bridges should be used for crossing land-units over it. But water-cities have their own rules that are not adequate to normal "bridge-situations".
If that terrain-modification would only consist of coastal-terrain, no land-units could cross those bridges.
Correct. You can allow the building of cities and even roads over water, but land units can't use them. So here's how I'd do it, incorporating Civinator's suggestions:
Let's use a New York map I've been working on as an example. I want to put in the bridges along the east river, especially the Brooklyn bridge. I start out by making normal land connections with LM Plains terrain:
As I start to find and modify the terrain files, I realize that there are more tile squares involved than I first expected. Moreover, I realized that I had to modify not just the lxpcd and lxdpc files, but also the xpcd and xdpc files as well. At least one of those little connector squares in the middle was found in the lxtgc file:
Here they are after customizing. I made a bridge (bonus) resource and bookended the Brooklyn bridge with it:
Finally, I add the invisible cities that will allow river traffic to pass the bridge and
voila! a bridge that will accommodate both land and river traffic:
obviously, that bridge "resource" needs work, but you get the idea...
The downsides are:
1. You can't run boats under the bridge unless you 'control' (have a city on) it. Foot units and wheeled units can use it anytime.
2. You can't allow the terrain you use to make the bridge to be irrigated or mined. Damn! should have used LM Desert terrain instead. Live and learn...
3. All cities for
every civ have to be invisible. That's fine for this map, but could be inconvenient for others.
4. Adding the city converts the railroad/highway underneath to look like regular road. It's not really a drawback, but something to keep in mind.
Here's an example of a modified LM terrain pcx for reference:
View attachment lxpgc.pcx
Whoop. Didn't mean to make a tutorial out of that post.....