Over the years, many railroads have learned that when their power could not pull all the tonnage up a grade, they discovered that their same engine could push half of the train while the other half was being pulled, therefore getting the entire train up the grade with the same power that could not pull it all.
The reason this is a fact was the study of friction the flanges experience riding the side of the rails on the curves. If the railroad was totally straight, there would be very little change in benefit, but most railroad grades always include curves.
Just one example was the Virginia Blue Ridge Railroad where I saw that in effect. Many coal short lines also worked that way as they had heavy trains for their power.
Often times they had a rail washer on the tender to remove the sand used for the driver wheels so the train would not bind up and stall. A train can be given a lot of resistance on a down hill grade by just heavily sanding the track and it would build up on all the wheels following and become a big drag without using the air brakes.