Railking GS-4 traction

Loco: Adding a little weight toward the front (in the smokebox area) might help.  Are the wheels clean, traction tires in good condition?  If you're working it that hard you might not be doing it any favors.  Are the front and rear trucks, drawbar installed correctly, with the proper spring tension?  If not ,they could be taking weight off the driving wheels.

Track:  Are  your "transitions" from graded track back to level constructed smooth and gradual?  If a long wheelbase loco struggles, it could be that your grade levels out too quickly and leaves the rearmost wheels with the tires hanging in the air.  Like the driving wheels, track should be free of oil and grease.

Train:  Make sure the trailing consist rolls freely.  put a tiny drop of oil on each wheel bearing.  If it's a passenger train, consider reducing the number of center rail pickups because they add a lot of friction.  Instead use tethers between the cars.  This will also reduce flickering.  You could even upgrade to LED lights (but this will not make them easier to pull.)  My $.02.

I'll see if there's some room for weight near the smokebox.

I don't think it's the track work per se, since the 4-8-4 is actually slipping on flat track.  It's just the rest of the consist is on a grade, and that seems to be too much drag when the locomotive hits the O42 curve.

How many/what type of cars is the locomotive pulling?

Dr. Chaos ,

I have the Railking Imperial UP 4-8-4 and I have a few O31s on some of the curves.  

The rear 2 drivers have the traction tires and I too had some issues with traction. 

I noticed the front six drivers would keep the two traction tired drivers levered only so slightly  from having the best contact with the track    I use Realtrax and maybe a different kind of track would work better.

I couldn't find any slightly thicker traction tires of the right size so I was bad and sanded away a little of the diameter of the front six drivers.     It worked.

Maybe removing  the front two drivers and replacing them with traction tire wheels would give it enough grip to help out the rear drivers.    I'm not that brave to be replacing drive wheels though.   😨

Replacing wheels on this loco would be difficult; a precision job best left to a machinist.  I'm not even sure that wheels are available for sale as separate parts.  You might get lucky and find a whole chassis at a good price, maybe as fallout from the upcoming MTH "inventory dissolution" sale.

If sanding the front wheels down worked for you that's great.  But personally I don't advise it.  If anything, ADDING to the diameter of the front wheels, even by gluing a tire on there temporarily, would shift the weight rearward giving the rear wheels a better grip on the track, and less likely to come off the rails at the top of a grade.  Making the front wheels smaller also creates the possibility of allowing the middle wheels (which are smaller on purpose) to snag the inside of the rail on a curve.  (Most of you probably know that by design, 8-wheeled steamers are kept on the track and propelled by their outer 4 wheels.)

If you have a similar loco that works and a dial caliper, you could measure the wheels (including the ones with tires) and see if there's any difference.

Ted , I wasn't very clear about reducing the wheel diameter on the steamer.

I had only removed an incredibly small amount of material from drivers 3 - 4 - 5 -6 .   I made scratches in the metal with 320 wet and dry and then polished out the scratches  with 2000 wet and dry.    I didn't try anything with the two front flanged drivers.        I didn't know if the rotation of the side rods or some poorly machined out of round axle journals (  very little run time ) were causing an issue but when I placed the engine on a flat smooth surface ,I could see a wee bit of rocking from the front to the rear drivers when pressing down on the loco.

I did look at the pilot and rear truck to see if there was a problem . I couldn't figure out if the four wheel trucks were an issue.       I have the RK Imperial Pennsy L1 2-8-2 and the Imperial RK Pennsy 4-8-2 Mountain  that have the 8 drivers with the traction tired drivers on the rear and dont seem to have the slipping issue even on the O31 curved areas.

I did think of using the ( Frog ? ) Snot on the front pair of drivers but I was told that I had better make really sure it was applied evenly and thinly or there would be a really bumpy ride for the guys in the cab. 🤕😵