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OK everyone. Now I show my lack of knowledge. My layout (n scale 3x6) is all in modern time and has a passenger and freight depot. I will be using F7s or F9s plus others and I am using DCC. I remember the terms "A unit" and "B unit". As I remember it, in model trains, the B units were just dummy units with no motors or the such. Now I see the A and B units with DCC decoders inside and also I have seen FT units (very long A and B units). OK, help me. Why do the B units now have motors and use DCC decoders for the FTs. I have a Athern F7 A/B unit engine and there is a motor in the B unit. WHY? The A unit runs fine on the track why would I want to add the B unit and get decoders for both?
Thanks for you help. This forum has been very helpful. Harlan......
The powered B-Units give you more muscle, just like the prototype!
For your 3 X 6 you probably don't need the powered B unit since you can't get enough cars on the track to use the power........but I run on a club and I can run 100 to 150 car trains if I want and you need more than one unit to do that. On the prototype, the letters designates individual power units.......for example Milwaukee Road bought their FT's as 4 unit sets, so the first units were numbered 40A,40B,40C and 40D. Frequently the A units had cabs and B units were cabless, but railroads (like Milw.) bought 2 cab units but ran them together as #A and #B.
As to why your F7 B unit is powered, I talked to a factory rep., and he said that because of the discounts they get for the quantity of motors and such they buy, having the unit motorized only added about 10-15 dollars to the price. At that point very few people would choose the unpowered unit since there is no harm in running the second powered unit vs a dummy. You are correct about having to buy a second decoder though.....but if it bothers you, it is possible to remove the gears and motor from the B unit and make a dummy.
Thank you all. That is the info I needed. I think, as mention, since I only have 3x6 and run maybe 10-15 car trains max, I will take the motor out of the B unit and use it as a dummy. Muscle is not a need for me. Again thanks all.... Harlan
You can also wire your B unit to the A units decoder, there by keeping both units powered. You never know when you'll need more power! (insert Tim the Tool Man grunt here). 
Rotor
You can also wire your B unit to the A units decoder, there by keeping both units powered. You never know when you'll need more power! (insert Tim the Tool Man grunt here).
Rotor
This would be my advice, too!
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