Hm, an undo button would be nice. I see your point. And yes, that tab is to control the amount of trains on that route
Just to clarify; the number of vehicles tab on the route tab would denote how many trains would run the route on a given pay cycle?
May I also suggest adding a City tab that states in short hand or icons what goods a City produces and buys, and a button to zoom in on the said city with a click. This would be really helpful at the start of the game when figuring out which part of the map to plant your flag and how to orient your railroad network for optimal gain.