Asymmetry

[Tyro]

I may be being stupid (again) but why do so many of my bridge designs work fine on the first pass (no broken links), but fail on the second pass (complete collapse). The bridges are symmetrical as far as I can tell. What am I missing?  The only difference appears to be the direction of travel.

[mikaiser]

For reasons unknown to me I never had this problem. If the train passes once it certainly passes again without any problems.

[tigerwalts]

It could be due to the bridge starting to oscilate on the first pass, then on the return pass some joints will be slightly more stressed.