Hi Ghost42.
This might help.
The vast majority of bridges used for running rolling stock are “tracks” rather than objects or spline objects. You must therefore be in the track flyout with the advanced dropdown menu selected.
A lot of bridges lay to the baseboard at track bed level, and don't look like bridges at all. You need to raise them to see the good bits underneath!
In the image, 1, 2 & 3 are the same bridge asset.
1. Lay the bridge and stretch it roughly the length you need. Sometimes it’s easier to lay it on a flat bit of board away from any complex hills.
2. Lift the ends after selecting the “Adjust spline height” button (a)
3. To get each end the same height select the “Get vertex height” button (b) and click on the raised vertex.
You’ll see a number appear in the “Height value” window (c).You can type your own metric value into that window if you want.
Click on the “Apply vertex height” button (d). That value can then be applied to the vertex at the other end of the bridge spline to make it level.
Stretch it as required using the “Move track” button (e).
If necessary you can “Insert new spline point”s (f) in between the two end ones which will automatically be set to the correct height. That allows you to create bends as well as apply gradients.
You then choose whether to build your terrain to the bridge, or vice versa.
Cheers
Casper
Imageshack