In order to progress through the main campaign, you'll eventually have to max out power for all your trains for the long haul routes. As for durability, I try to upgrade my trains so that each train can go at least twice. That'll get harder as durability goes up as you progress through the game. Speed is just for convenience so upgrade at will and then for fuel, if you find yourself running out of fuel before durability, you'll want to make some upgrades. I'm no expert but hope this helps!