Guess I was wrong about the mail, it just seemed that way to me. So no, it's not the generation, but rather the revenue that's double with mail for some reason, this is strange, if you compare northern lights lines Ayl-Dun Mail and Aylstone-Dunding, you'll see the stats are pretty similar, while revenues are double for mail.
I think choose signs are especially required in the pre-rail era, since you have to rely so much on slow road traffic, even with no spacing, just the short loading time in the station can be a major bottleneck in long lines with many convoys, although this may be because I still havn't figured out exactly how to space them out properly with the large lines.
You can see refunds on some of my lines now (hunderds of dollars), but they are far less substantial than in 0.8.3, where a large network could lose huge amounts within a month.