I have added sound to several Bachmann FT units.  It is not difficult, but it does take some preparation work.

I use an MRC 1862 decoder or one of their new 1909 decoders , but it should work with whatever decoder you want to use.

Step by step is as follows:

Remove the shell from the B unit.

Remove two screws from pc board. 

Gently push the pc board toward the back of the B unit.  (it does not matter which end of the chassis you pick, but I prefer to have the sound on the end nxt to the A unit.)  Then grind off the mounting peg (and headlight holder if necessary) to make the mounting surface for the speaker as flat as possible. 

Carefully grind off the back of the speaker enclosure until it fits below the shell of the locomotive.

Once you are certain that the speaker will fit under the shell, silicone it to the locomotive on the surface you just prepared.

Solder (A) the jumper wires between the pc board terminals for rear wheel pickups and the front wheel pickups making sure to include (B) the decoder inut wires on one end of the pc board.

Once your soldering is done, program the decoder and test the locomotive.  If it works, suspend the pc board in a tape cradle to keep it from rubbing and creating a short.  I use clear tape, but electrical tape might be just as good.

Put the shell on and you are done.

Its not going to be Broadway Limited or Proto sound, but it works for me.

Actually,  Santa Fe Warbonnets were passenger locomotives and were ALWAYS run with a B unit because the A unit did not have a steam generator to supply passenger cars.

I have several AB sets and two ABBA sets.  These locomotives seem to work well together.  Unless a decoder goes bad.

I just purchased a new Bachmann HO ATSF Warbonnet F7A (item no: 63003)  It looks great and runs really well, but it is light.  Does Bachmann make a B unit to go with this A Unit?