The DLS has several thousand bridges, many of which are extendable splines.
Most have a surface of some kind, usually with a road or rail track surface. There are some which have cobblestones or footbridge type surfaces and others (which I can't immediately recall) which have open girders ready to lay a track.
You can always lay a spline surface of your choice over any bridge if you can find the right choice, and size, for what you are trying to build.
What exactly are you after?