Williams F-7 AA. Southern Pacific model # 4307 Both A units have dual motors.

First question? how steep of a incline do you have? As to me I would think one engine should be able to do that job. try cleaning both your track and the wheels on the engine. 

DOUG , MOTORS ARE PROBABLY WIRED IN SERIES . WILLIAMS RUN SUPER  FAST  WHEN WIRED IN PARALLEL

Any single twin motored williams will have no problem pulling those cars. 

I wonder how many miles are on those engines?  I bought a worn out EP5 that didn't run the greatest, the flywheel motors had too many hours on them.

I put on two locos that I hadn't run for some months and for some reason they became out of sync! They fought each other until I realized what was going on. Without the modern electronics I think running multiple motors is problematic and as noted above how they are wired can affect performance. If you can tether them all together it may help. I have an A-B unit that I have done that with and it cured the stuttering.

Most of the electronic E-units made after 1990 would start in forward if left off more than ten-fifteen seconds. If the one running in reverse is pulling against the lead unit and your E-units are the start in forward type then reverse the motor leads on the trailing unit and they will stay in sync.  This will not work for conventional mechanical E-units or first gen electronic E-units.  I think all the Dallee E-units do the start in forward trick and would get the job done for your two units.  OR you could run jumpers back to the trailing unit and run all four motors from one E-unit however you need to add some overload protection you may try 4A PolySwitch resettable fuses on the output of the E-Unit.  If they trip too easy try 6A. You can find these cheap on eBay.

Really good info here.  Thanks for responding.  Doug

I have a large run up to the mountains.  The incline is at 3.5%.  They slow down and cant make it the first 120 inches as it reaches 3 inch height.  3 over 120 is 2.5%.    Dual AA pulling 5 Williams SP Coaches and they cant pull em.  The rest of the way is another 100 inches to 7 inch .  7 over 200 is 3.5% total..  Lionel gp 38's fly up it pulling 15 cars.   I'll work on all the suggestions.  Thanks  again.

try it with just one engine