Wow! I don't know why Bachmann doesn't emphasize the difference in the curve radius.  You 've got me thinking that the reason that the F-7's I have work so much better is because I have the 18 inch or shorter curves.  This may be why the AHM and the Ahearn's I tried didn't work so well on my layout. 
Now that I believe the reason for the better F-7 operation on my layout, I'm convinced that there is no changeability with other diesel shells for the F-7's. Therefore, on the second note, I'll rid myself of the F-7's and change the curve radius on my layout to support the larger engines that I really like much better.  Maybe I'll keep one or two for switching yard.
I'll keep you posted on my changes...  I'm still in the process of trying to retire...so these changes won't be immediate...     Happy Holidays to ya, and Thanks!

Let me get this straight, are you saying that the F-7 and F-7A frame and motor (not the shell) will fit in the shell of the GP40, GP38-2 or SD40-2?  When I measure the wheel carriage of the two, the GP40, GP38-2  wheel(carriage) base is longer than the F-7 wheel base.
And Yes, I'd prefer the more common freight trains, I'm trying to look more modern, up-to date, on my layout and in this case the engines too.

P.S.  Thanks for replying so rapidly, I really appreciate this.

I'm more of a freight type engine guy.  The F-7 and F-7a really look like the Superman stopping train,  (from the 1960's TV show) and I really have low interest in this model engine.  Bachmann's engines in this model perform very well on my layout, I've had friends use their AHM and Ahearn models and they didn't do very well on my layout. I'm looking to buy a different Bachmann diesel, DCC of course, but I believe that the length of the engine, it's wheel carriage, is the factor that reduces the performance of the engine on my layout.

The F-7 & F-7a  normally numbered 307 I want to swap the shells with some other diesel shell.
Or would it be easier to sell the F-7 and F-7A DCC and buy different models. The reason is- these engines ( I have four of them) work so very well, I'm not good at re-painting, and they are getting confusing as to which one I'm working with. Also buying the shells would be cheaper than replacing the entire engine. Any suggestion would be greatly appreciated.

Okay, could you clue me in on Smart Devices?  From what I recall smart devices are on phones, the App is downloaded to a phone.  Please tell me that there are smart devices and Apps that do not involve being attached to a phone system!
Basically, Are you telling me that train sets are now going to have to have a phone attached in order to operate model trains?  This means that everyone everywhere will need a phone in order to use this new EZ App?   Can this EZ-APP be used on a phone that's not connected to a phone company?  Say without a SIM card.   If that's the case, then I'll gladly move to operating my trains on one of my old smart phones that isn't on the phone companies rental/usage charge.
  I'm all eyes (Ears are for hearing), eyes are for reading your response.

Hate to be a stick in the mud....
But Would there be issues even if I used track separators to separate/isolate each train set from the other.  Yes, the engine would probably have a heart attack  (short as it crosses from one track system to the other) since the pulses aren't synchronized.
Basically, what I'm asking is track separators also won't work help the condition too?

Okay, could you clue me in on Smart Devices?  From what I recall smart devices are on phones, the App is downloaded to a phone.  Please tell me that there are smart devices and Apps that do not involve being attached to a phone system!  I'm all eyes (Ears are for hearing), eyes are for reading your response.

Question.  If you have two train sets (both identical DCC train systems), can the two train sets be connected via the train track and not cause the EZ controllers to fail, since both are pulsing square wave DC with a continuous 1 amp on the tracks.  Similar to AC systems, I could devise a synchronizing circuit, where both pulses from each EZ controller are timed exactly the same.  Basically I'm looking at whether anyone has tried this with success minus the synchronizing circuit to connect two train systems where the tracks are connected without adverse issues.. 

This is related to a previous post: Can two (2) EZ Command Controllers Work Together? 

Interesting!
I had to really think about what you indicated about the other systems only being a modest additional cost and the fact that EZ command is a obsolete system, with no upgrade path.   From this point in my modeling (Remember I'm only an intermediate type modeler, and probably only for the kids) moving toward a more detailed system, Dynamis being one of them, I have determined that moving toward the more involved train controlling systems which is somewhat more involved, would definitely decrease my kids interest.  Also note that my kids are still bombarded with Playstation, Xbox, etc. As for me, reverse engineering is kinda what I do for a living.  That's because the Power Industry, electric utilities are running into the same issue, old equipment that manufacturers do not support, equipment thats more than 20 years old, but the equipment is still highly reliable.  The utility will invest in and reverse engineer obsolete power components.  Sorry got of the subject.  My kids and me too, like this EZ command, watching videos (U-tube) on the other systems seems a bit cumbersome, larger controller, or smaller controller with lots of buttons, which means lots of options, which means more experience needed, or longer learning curve.  The EZ command not so much.  Lastly, watching my kids play with this EZ command and if they continue being interested in models trains, I'll gladly waste my time trying to reverse engineer the controller, probably do to others things.  If the kids loose interest, maybe the trains will find themselves on E-bay.
Thanks for making me thing about this, only the kids will make the final determination.

I believe your doubts are correct.  I'm gonna try asking anyway.

In addition, so far everything I've read, leaves me to believe that the Bachmann control is the simplest of all the DCC controllers.    The fact that Bachmann had a product that would support two controllers is exceptional.   The other manufacturers may have the ability to support additional controllers, but their expense is most likely outside the Bachmann market where the consumers are mostly younger less mature modelers.  Bachmann seems to be in the younger less mature market.  Their simple controller should be a much easier less expensive way to bring two or more controllers to those in the intermediate range (that's where I am at the moment).   

I am surprised that the older mature modelers haven't notice that the Bachmann controllers could upscale those more expensive controllers.   I also believe that Bachmann is missing a great opportunity to market to us intermediate range modelers.

PS.  Thanks for the post on Model Railroader

You've just made this a challenge.  I'd like to reverse engineer one of the DCC controllers, but to make it easier, it would be nice if you or someone has the schematics of the EZ Controller Model 44908.   So I can modify the controller to work as a walk-around companion.  I don't think the Bachmann Co. would freely give up their schematics to either controller.  They may look that as proprietary.   Having the program language would be a plus but not necessary.

Here is the scenario:  Two kids wanting to run their own trains.
Problem:  I read the last discussion (2013) and its reply stated to obtain the Walk around companion controller (Model 44907), then I read that the Walk around companion module was discontinued (2016).
Question:  The last discussion didn't go into detail so I'm presenting that detail.  If I connect two EZ command control centers (Model 44901):  One as the main connection to the track and power supply, then connect the other command control center without connecting the power or connection to the track can this support operating two trains where each kid can control their own train from each control center?  Basically providing an individual controller similar to the walk-around companion which currently I can't seem to find.

Mr. Maletrain,  You burst my bubble.  I Was really hoping that I could swap out the my approx. 5 year old EMD F3A with the GP, cause then I would have my entire line representative of BNSF.  But as a side note, I will still try to swap both and see what comes of it.  I'll have to wait till tomorrow when my first DCC locomotive shows up, and then my question and anyone else that's curious to find out by physical fit.   

By the way, Thank you all for responding so quickly, I didn't expect to find any responses before my DCC locomotive shows up at my front door.

If I understand you correctly, the chassis of the DC model and the chassis of the DCC model are the same.  The DCC controller doesn't take anymore space on the chassis as the DC model?   If that's the case, then I should be able to move to replace my EMD F3A shell with the GP38 shell also.   The EMD has the rounded face, and the GP is well... square.

You stole my expectation.  I haven't received the DCC locomotive yet.  Amazon indicated that it should be here tomorrow.  I will exactly try  it like you stated to see if there are any interferences.

I thought If I asked the question on this forum, that someone had already had experience with swapping DC shells with DCC shells.  Again. I hope the shells are exactly the same.  I've seen where some DCC locomotives have a opening on the top of the shell case to get to the control module.