First off, I have included the original (or what I believe to be the original) KML for polyline that would be the Hebert C. Bonner Bridge south of Nag's Head, North Carolina.  If you'll notice, it lacks any sort of Z axis value for any of the points.  Just drew a nice, multipoint line like I'd be drawing a road.  I know of no limit to the number of points a line can have, but in the thread were Doug Matthews so patiently

The next file of note is bonner_bridge.txt.  This are the points of the polyline after being edited with FSX_KML's vertex editor.  Originally I had intended just do this by hand but the vertex editor streamlined that for me.  Sadly I lack the modified KML, but luckily retained the list of points with their height values.  These are in feet (my original test bridge over a river in Maryland used meters, so both work).  A bit of explanation: the two end points are negative values to create ramps.  If you put them above ground, the top of the road surface will be above ground and the bottom of the deck will be at ground level, creating some weird tube-like structure that is invisible from the inside but visible from the outside.  That sounds vaguer than I thought it would, but you probably understand what I'm saying.  The varied height of the points is to create the hump the bridge has to accommodate pleasure and small fishing boat traffic between Pamlico Sound and the Atlantic Ocean (you are receiving a hidden geographic lesson tonight).

Thirdly, we move on to bonner_bridge_xml.txt.  This is the text file where I shoehorned my coordinate list into the known working XML.  You can study this to see where and how you'd put your coordinates into an extrusion bridge's XML.  I don't think the order of latitude and longitude matters in your coordinates, but if you're first point has lat first, keep it that way the rest of the file.  Otherwise, it wouldn't compile.  At least it wouldn't for me.  

Then we have the XML.  It works, it will compile, and you will have a bridge.  

Also I have included my finished BGL and the exclusion that gets rid of the section of the Bonner Bridge that was in the default FSX.  If you want to see it in action, start a flight at Dare County Regional (KMQI, I think) and head east a mile or so to the Atlantic Ocean.  Follow the coast south and a few miles later, you should see the bridge on the western side of the narrow barrier island.