Well good Dr. - I think you wandered down the wrong track on this one.

The weight of the loco directly correlates to starting tractive effort, but is most cases has little to do with dynamic braking effort. I have been on a loco that slid in dynamics, but the rail conditions were extremely poor. Typically, a loco's dynamic braking effort is well within its normal adhesion limits.

As pointed out in another reply, the SP SW8s are lighter than a typical GP7, though in this case it makes no difference other than the heavier loco will have a proportionally higher starting tractive effort. The size of the prime mover generally makes no difference as to dynamics, even some slugs have them.

My question related to the capacity and resulting retarding effort of the dynamic brake grids in a SW8. EMD parts manual show units first being equipped with 540 or 600 amp grids, depending on the supplier, then later 700 amp grids. According to the parts list, it looks like an SW8 has 540 amp grids.

The 700 amp grid stayed standard for EMD into the Dash-2 era. It is geneally accepted that these were capable of creating a maximum of 10,000 pounds retarding force per axle (depending on speed, gearing and for later GP35 on if equipped with extended range). Since the SW8 and early GP7/F7 only have 540 amp grids, are they proportionaly lower in retarding force? EMD went to a high-capacity grid that bumped the retarding force up to 12,500 pounds per axle. I believe ACs are above that.