How to get more traction

Add more weight to the locomotive.  Also, sometimes the pilots of a locomotive may force it to unweight the traction tires and cause it to loose traction.  Truthfully, the Berkshire JR models weren't strong pullers anyway, but they should be able to pull more than 5 cars.

The heavy tender is actually a liability in this case, since you are lacking traction.

I've found some steamers with traction tires don't pull well on certain types of track, I have a Williams brass K-4 that sucks on Fastrack, but pulls fine on tubular track.

Try Bullfrog Snot. Apply a thin coat over the traction tires wheel face. You can put it over the tires and all, then let it dry for about twenty-four hours.

It's a little pricy, but it works.

Another point is John is right you may need some more weight in the engine. Most hobby shopes have sick on led weights for this purpose but finding a place under the shell is the trick. 

http://bullfrogsnot.com/

Have fun.