[48"x18" Special, Complexity 8/10]
This is an opportunity to provide an access point through a closed loop railroad. To achieve this, a bascule bridge can be raised (even weighted to remain open except when trains transit) for passage to the opposite side.
Side supports that bolt to the module ends would allow for more compact storage and transit. A removable bolt serving as the bridge hinge could be incorporated as well.
Zig Zag Gorge
[48" x variable" Special, Complexity 8/10]
Here is a module that is both difficult to construct and challenging to navigate. Multiple levels meander through and around a deep gorge, hairpin turns, and bridges galore...
Stiffening this module takes some creativity. Consider using hollow aluminum or steel rectangular bar to provide additional stiffness within a wood box as shown. This would provide the necessary strength and stiffness since the height of the module narrows through the gorge.
You may also want to incorporate metal bar surrounded by wood in the bridge decking to ensure they are sufficiently rugged and robust to withstand leaning children. The segment top could be slanted into the gorge for an even more striking presentation.