I was successful with installing a Digitrax  SDXH166D.
I loaded the Digitrax Traction-V1 sound project in to the SDXH166H with a Digitrax PR3 decoder programmer. I made small simple modifications to the Traction-V1 sound project to turn off the compressor and whistle. I used two squared off Red & White 12vdc LED's glued together on both ends to get Red & white lamps on both ends depending on travel. I kept the 12v incandescent lights in the cab as these were nice and bright.
I did my best to hide everything under the floor boards and used double sided tape to hold it all down. The square speaker that comes with the SDXH166D is mounted in the center of the car on top of the floor boards. I'M not too happy with where I mounted the speaker, having the speaker wires exposed and only having tape holding down the floorboard. It works and the sound is good but low volume. I would post pictures but I haven't been successful with attaching them here. It will be great to have this trolley on the Christmas layout this year. Thanks.

Flare, think about replacing the bulbs in the ceiling with 12VDC 3mm LED's. No resistors.
The existing light board will have to go of course and connection made to the decoder.

Thanks for responding.
I'd like to try this myself.
I just added a DCC Tsunami 2 decoder to a Kato HO Chicago Metra F40PH.
That went surprisingly well.

Friends, I have a Bachmann 25127 Christmas Closed Street Car that I want to add DCC.
If you have experience with this can you suggest;
* a decoder that;
   - fits well (possibly describe how and where to mount)
   - is of the proper current for this Street Car
   - has sound capabilities for this era streetcar.
   - can control front, rear and in car lights.
There seems to very little info on the net.
This is a great Streetcar for a Christmas village and adding DCC puts it over the top.
Thank you for any help you can provide.