As strange as it may sound, tracks last longer if they are used. Rarely used tracks are in better condition than tracks, which are not used anymore. It have something to do with rust and metal wheels removing rust. Sometimes real railroads sends trains over rarely used switches to remove thr rust. You may have tried being in a train, which went to the left track and returning to the right track right away. Now you know why
Rust reduces top speed (and increases the brake distance very much!)
It haven't got much to say about max weight for slow trains, if the rust is not too great.
And about trees. Do you think a 100+ tons engine cares about a single tree
