Some great input and advise given here. Tractive effort varies greatly on locomotive models, depending on weight wheel make up, yes different profiles and materials make a difference . I have 4 axle units that would drag a 6 axle unit in a test. Models can be very finicky.
4% grades are gonna be a challenge. Just as info, the timetable I still have for the D&RGW/SP Minturn to Tennessee Pass (3.5% grade) a GP38 was good for only 460 tons, so with a loaded 130 ton car, means that a GP38 could pull 3 cars. A GP40 was good for maybe 5 loaded cars, MT cars weigh in around 30 tons, so you can do the math. SD40T-2's w PTC (positive traction control) were good for 1200 tons, an AC was good for 2100 tons (I think they are waaay over rated myself).
As for adding help,(pushers), you should not shove on the end with more than one powered unit. Adding swing (mid train) help is almost always better. The challenge comes as to where to cut the swing help in, they should be pulling as much as pushing. There are many other factors involved if you wanna go all nuts with the not shoving on hazmat on and on.
As always rule 1.0 applies, as it is your RR and what ever you want to do is OK, rule 1.1 reefer's all other issues to rule 1.0. I am trying to show you there is almost always a prototype to match a model. I knew a guy that had a model RR that used swing help on every train over 15 cars, just for a 4' shove, in HO scale. Hey, his RR right?
As for your track power issues. As has been said before, a lot of issues can come to play as well. Is your trackage mounted on solid roadbed, meaning not on movable risers, or not supported between joiners? Is your outfit operating on carpet, or outside? Do your locomotives have all wheel electrical pickup? Are all those connections working properly? I am not that much knowledgeable about N scale. But many of these principles cross scale lines.
I am not at all trying to run down Bachmann, I dont know what vintage your power is, some early N/HO scale Bachmann power can be a challenge as well.