All I can tell you is that automatically when the train climbs the grade it will have the throttle voltage,whatever it is set at. When the train is on the layout it will have 3 volts less than the throttle. When it goes down hill on the grade it will have 6 volts less than the throttle. These values are adjustable to meet needs.
I can not comment on the engines ability to pull cars at a given voltage. Sounds like the Lionel loco is not good quality or there is some other problem. Most any Railking or Williams steamer or even a PW Lionel 2055 will pull 6 cars up a slight grade so i am not sure what is going on here. Seems like this is an issue with the loco and not voltage input. Perhaps someone who has that engine will comment on performance.
I am not sure also of the tap configuration on the CW I never had one. Maybe someone can provide a link. I need to know the voltage of the accessory tap and which terminal on it is the common with it. I think this is backwards for PW type Lionel locos. On a PWZW the common is "U" for example. The CW should have enough power to run the train if you do not have a whole bunch of accessories hooked to it. The 3 relays will only draw 2 watts of power or less. I also assume you have adequate wiring and feeders. How big is the loop and how are you wiring the loop.? Your lack of pulling power could be due to voltage drop,dirty track joint etc.
Just to add after reading your post again. Putting the smoke unit on should not slow the engine,unless there is some design defect or it causes a voltage drop to the track which is not wired adequately. If this happens and it is not a wiring problem, I would get on Lionel's case about it. 90 watts should be adequate run a 1 engine can motor train and a smoke unit and even a few lighted cars.
Dale H