Around how many freight cars can a GP7 realistically pull?

[nachoman]

on level ground I have seen early geeps pull quite a bit. But on a 3% grade, much fewer. The grand canyon railway has a GP-7 that they use for ballast and work train service. My brother once told me that a few of their ballast cars rarely get used because the GP can only pull about 4-5 loaded cars up a 3% grade. The remaining cars are stuck at the end of a siding and never get out.