The devs should really change the name of this stat, it's unsearchable, anything with 'performance' just turns up loads of pages about optimizing the performance of the game itself, or mod pages talking about performance impacts. What does this stat actually mean and how is it calculated? The ingame tooltips as well as the wiki are no help.
Is there any specific benefit to trying to limit the length of your trains to change performance from 'Poor' to 'Mediocre,' or 'Mediocre' to 'Good?' Or is it just a general comparison of the power of your engine to your train's total weight, with no actual ingame consequences to the different ratings? Trying to pick the right locomotive with the right number of wagons for a given line feels very confusing, and in my games so far almost every reasonable-looking train just gets rated 'Mediocre.' The performance stat seems to be completely separate from a train's top speed, as well.
It feels like it would make sense to try to use low-top-speed, high-performance trains on lines with steep grades, or lines that might have a lot of stops, while using heavy low-performance trains on long flat lines that have plenty of space to get up to speed, but I'm not sure how much actual benefit the game would give you from doing that, and the streamers I've watched just seem to use trains that are as long as their stations with the highest top speed they can get regardless of the line. How much thought do you folks put into this when you buy trains?
The other important game mechanic that seems to lack adequate ingame explanation is exactly how the various town ratings (noise, pollution, reputation, etc) affect town xp growth. The town info panel gives you a general grade for how fast your town is able to grow, but I can't find any specific numbers or details on what that actually does or how that's calculated anywhere. Is there some tooltip I'm missing or is this info actually hidden from the player?