I've always liked the look of trestles, wood or steel, so it's always in the back of my mind when I'm designing a track plan.  I don't remember exactly when I included the trestle when I was sketching the plan out, but having the highest track in the middle of the three on a fill would be visually awkward.  It would make the center of the layout seem like it's in the middle of a "bowl", and block some of the views to the interior of the layout from the primary viewing position (whether the layout was in a coffee table, or in a corner).  A low trestle in that location opened it up, make it seem less confined, and I get to include a long trestle on my small layout.

I wanted to have a more modern steel trestle than a wood one, so I naturally chose to use Micro Engineering's bridge pieces.  I used the thinner, shorter 40' bridge sections, and avoided using any of the 80' sections, as they would seem rather overbearing on a small layout with short cars and engines.  Using all 40' sections would also allow me to make the trestle look more "spidery".  Once the subroadbed was completed, I had the actual length of the trestle.  Of course, it wasn't a fit for full 40' sections, and the creek required a slightly longer span, but I only had to modify two sections.  The picture below shows a standard assembled 40' span in the middle, with the longer 50' span to cross the creek to the left, and the 25' foot span for the West end of the bridge on the right.

Next, I built the end foundations out of styrene pieces and painted them "concrete".  I temporarily placed the spans on boxes and miscellaneous items, and laid Micro-Engineering's code 55 bridge flextrack, soldered each end to the Peco track, and then applied ACC to the rails to lock the flextrack into place.  Then I removed the temporary supports, and installed the spans on each end by ACCing the top of the span to the bottom of the track.  I then applied spans one by one, working my way from each end towards the center.  Once all of the spans were glued into place, the bridge was able to support itself, without the supports added, as you can see in the picture below.

I then modified the 80'+ high supports to the right height for the low trestle, and further modified the support on the West end to fit on the side of the hill.  I then used styrene for the concrete bases.  I then added each support so that trains could be run on the bridge (it could actually carry cars without bridge supports, but not engines... I tried).  I didn't include the X-bracing at that point, as it would have made it very awkward to add scenery under and around the bridge with the X-bracing in place.

The hillside was filled in when the rest of the hills on the layout were built, and ground cover and ballast were also applied when that was done on the rest of the layout.  At this point, small trees and shrubs will be installed, and then the bracing will be added (except for the long span where the bridge crosses the creek).  So as not to create the visual "bowl" I want to avoid, the foliage will not be higher than the inner loop tracks.

Like some of the other scenes, I have the rest of the structure pieces on hand, so like the grain and fuel depot scenes, this will be one of the earlier scenes to be completed.