i managed to get it into the track, but i don't know why
after having tried several things, i changed the key-colour to ff00ff (why does pink work and blue does not!? not very logical), after then was the first time i saw the bridge at all, but it had a tree texture so i had a look for this object in the track.ac file, edited tree1.rgb to the bridge texture filename and voila. i tried it as rgb and png, but everytime i run trackgen, it picks this tree-texture (even after i kicked all trees from my object map, so that there should not be any tree textures at all).
having this in mind, i recognized that all the other trees also use tree1-texture instead of their own (tree2 etc), i always wondered, why they look so similar ... so what is wrong with trackgen, so that it always uses tree1.rgb for each and every object!?