Around how many freight cars can a GP7 realistically pull?

[Puddlejumper]

I think I'm going for 5 freight cars being towed by a single GP7 on a flat layout. Would that seem "believable" in terms of what the real GP7 can pull?

I'm certain a GP7 could pull five freight cars.

Definitely. I'd venture to say that, under the right conditions (dry, clean rail, etc), a GP7 would be good for 50+ cars on a level surface.