Your problem is fairly simple in nature:
You are utilizing an analog DC controller while locomotive is equipped with a sound decoder for digital command control and is optimized for use with a DCC system/controller.
You have three solutions as I see it.
1. Upgrade to a DCC system whether that be a Bachmann product such as the EZ-Command system, the Dynamis Ultima (more readily available in England) or any other DCC system of your preference. Note that other companies have their own proprietary systems that only have limited DCC capabilities such as MTH's DCS.
2. Remove the sound decoder and speaker and install jumpers.
3. Be content with the status quo

Quote from: Razmataz on May 15, 2018, 07:45:01 PM
I am grateful for this forum! I don't know where I would find answers at if not. I just bought my first train set with DCC locomotives. EMD GP40 and EMD FT-A locomotives. (by the way I have no idea what those numbers mean) I would love to know how and what sound systems are compatible with these. Also maybe what number or style of couplers they use. I know they are knuckle couplers but I've seen several makes like kadee but there are different types also

The Bachmann Digital Commander Train set includes two Sanata Fe diesel locomotives the GP-40 and FT-A which are both equipped with Digital Command Control decoders however neither have sound functionality. Adding sound would cost about $150 per locomotive depending on what sound decoder and speaker you utilize and other factors. But as is they have no sound and would not have any sound regardless of what DCC system/controller you utilized. Coupler numbers only reference Kadee brand, couplers are often described using other terms for all other manufacturers. The locomotives have Bachmann EZ-Mate Mark II center-set medium-shank couplers and any HO operating center-set standard-shank knuckle coupler would work fine (this includes Kadee #5 and #148 as well as other competitors to Bachmann EZ-Mate Mark I & II). Generally most locomotives and cars from most manufacturers use center-set medium/standard-shank couplers. The couplers are included with the locomotives if you are somewhere other than USA or Canada (i.e. England, Australia, Germany, France et al) then you made need to convert them to use with other locomotives and rolling stock

Quote from: cmsluss on May 05, 2018, 04:01:48 PMHow does it perfrom on 18 inch radius curves?
How is the DC performance?

You really will need at least 22" radius, I built a test loop of 18" radius to check and mine continually derails, especially the lead trucks, trailing trucks and tender, the drive wheels do ok.
Analog DC it runs fine but the start voltage is much higher and you will have no access to non-automated sounds.

Quote from: dennislee on April 27, 2018, 06:35:12 PM
what is the process to repair a steam engine? over 25 years old. I have already sent it in.

If you pay the service fee you will probably be offered an analog 0-6-0 to replace it or possibly an analog 4-axle diesel or analog trolley.

If your item is not under warranty then their policy is a bit different. It depends on if you were the original owner, if it is within one year of the date of purchase, if the locomotive was modified, if the locomotive is still in production etc.
Without knowing the exact details it is hard to speculate.
Did you pay the repair/servicing fee of $45? Without paying that your locomotive will not be serviced or replaced.

HO scale is 1:87th while O scale is 1:48th, the trolleys you listed are 1:87th which would be too small. O scale would be too large.
What you need is S scale, which is also 1:64th scale, I think American Flyer made a trolley at some point in the scale you are looking for.

Keep in mind the tender that shipped with the 2-8-0 and 2-10-2 varied by roadname sometimes. I have a Bachmann 2-8-0 with a Vandy tender and one with a USRA tender for instance.
Also GP 40 shells are interchangeable (last I knew) with other GP 40 shells but they will not fit on a GP 38 correctly and vice versa.

To answer your question, you can install a DCC decoder but it wouldn't be worth the time and effort or the money. You can buy a number of n scale Bachmann locomotives with DCC at a very reasonable price and they are better in every respect than a 30+ year old GP 40.

What is the item number of the locomotive and when did you purchase it? (assuming it was new)
What decoder are you using?

Sir, I am not the Bachmann but I can tell you that Bachmann does not specifically produce parts for separate sale. Bachmann gets their parts through the normal production process, so when a locomotive is run there is usually a surplus of some parts that are offered for sale only because when the production facilities run it is hard to make the exact same amount of every component and part that is used to make a locomotive. Until the locomotive is re-run the parts will not be available. However, Bachmann does offer an excellent warranty on their locomotives so if you recently purchased a brand new Bachmann locomotive and it requires repair or servicing, Bachmann will generally exchange the locomotive for a new one free of charge with a copy of the receipt, warranty card, etc.
If you have older locomotives that are not covered under warranty, then my best advice for you is to either upgrade to the newer models which are quite a bit better all around than the older models or patiently wait for several years for the locomotive to be re-run and additional parts to be available.

I think there are several very tiny screws that hold the shell on actually if I am not mistaken.

Bachmann does not make any add on sound only decoders for N scale, they are only available in other larger scales such as HO and On30.
If you wanted sound, your best bet would be to buy a new decoder entirely that includes sound as well.
Even if Bachmann made an add on module of some sort, it would do you no good because none of the non-sound DCC locomotives made by Bachmann include a speaker.
Keep in mind space is often the limiting factor when dealing with N scale so often times installing sound could be either very difficult or even impossible if there simply is not a sufficient/requisite amount of space for all the components. Some locomotives such as the DD40AX which are quite large compared to other locomtives are much easier to work with when installing sound.

Quote from: ACY on March 21, 2018, 03:57:17 PM
Try the Baltimore Society of Model Engineers
225 W Saratoga St, Baltimore, MD 21201

Or The Severna Park Model Railroad Club
Severna Park, Maryland

Or The Baltimore Area American Flyer Club (S scale sorry)
Havre De Grace, MD 21078

Also this website lists quite a few in the Baltimore/DC area:
http://potomac-nmra.org/local_interesting.html

Try the Baltimore Society of Model Engineers
225 W Saratoga St, Baltimore, MD 21201

Or The Severna Park Model Railroad Club
Severna Park, Maryland

Or The Baltimore Area American Flyer Club
Havre De Grace, MD 21078

Also this website lists quite a few in the Baltimore/DC area:
http://potomac-nmra.org/local_interesting.html

1st Bachmann never made a 2-12-4
2nd No 2-12-4's were ever made for the North American market
The only real life 2-12-4's were ordered by the Bulgarian State Railroad as seen in the photo below. Only 20 were made.

Photo by Ivo Radoev

Union Pacific however made 4-12-2's and they were made in HO


If you meant 2-12-2, only 2 were made (in 1941) for standard gauge in real life and were operated in Austria by the Deutsche Reichsbahn.

The only locomotives Bachmann made that I know of close to a 2-12-4 is the 2-10-4 and the 2-10-2. Are you sure you don't mean a 2-10-4 or 2-10-2? ...because it definitely was not a 2-12-4