Pull how many cars?

IMO there is no way 10 cars should do that.  Even if they are all diecast cars. 

Hi do you oil your trains at all.

 if you don't oil he cars one car can = three to four cars.

 then those ten cars can be like pulling therty to forty cars.

 found this out a long time ago.

It depends on the loop on my layout.  I have one loop with all 36" curves and no straight longer than eight feet.  I like to run no more than eight cars, counting a caboose, on that, and run only shorter locos and cars, at that.  It just looks good. 

My other two loops are 54 and 72 inch curves respectively and have straight sections over 20 feet long in places.  I like twelve to fifteen scale cars including caboose if freight, or or seven to nine 15" or 18" passenger cars.  That looks best to me. 

Absolutely not  That SD70 can pull 40 cars with no problem  What I have seen the most that causes this problem is that you pick up some type of foreign material in the gear train and the motor craps out from the bind.  Things like ballast staples etc can really bind it up. 

Here is an MTH Q2 pulling 35 cars up and down some serious grades

I have oiled all my cars and oiled the train to be sure all it good.  I have a pretty large floor layout that goes around the living room with many 10ft+ straights.  Under the tree is a tight section that makes a X reversing loop that limits the haul to 10 cars.  Seems that the you all think that their should not be a issue, thus it must be a bad motor.

I do have one MTH window display car that does not roll well that is not used much.  Any tips, other than oil, to make it run without binding?

Bill

Make sure both motors are running in sync. If the rear motor and the front motor are slightly off, it'll drag on the slower motor and burn it up.

10 cars isnt the cause of your problem. I've seen Premier diesels pull 50+ cars without much trouble.