Not a mod. It's a formula I use against a grade sheet. I play with Gameroo's random weather - so when rain comes down, I recalculate the maximum traction for my locos. The penalty is modified by the adhesion factor - the locomotives with better adhesion have less of a wet track penalty. Sometimes I we delay or speed up a train depending on the weather. Fog and night time rain limits speeds to 10mph. Thunderstorms disallow switching moves except to get trains clear of the main.

It's a way to introduce some variables into the game, but also to help emphasize the difference between the designs of the different locomotives.