While the subways are generally fine, the short tracks between buildings often cross caves or lakes with no supporting blocks beneath the track. These track pieces break, leaving behind an item. This is often a lot of items and can seriously impact performance.

minecraft 1.15.2
forge-31.2.31
lostcities-1.15-3.0.1-alpha.jar

I have no idea how to fix this to be honest. The lakes are generated later and when they generate they don't properly clean up tracks above them

Could you maybe clean up any item entities shortly after world gen is finished for whatever chunks you've touched?