I'm assuming that your Athearn SD9 has a MIR 071 body fitted to it if so I use a Digitrax DN135D its a great little decoder and has a lot of lighting effects and unlike the Hornby/Bachmann decoders you have a lot of CV's to finally adjust the motor. But most of all it is small in size and therefore you should not have a problem fitting it into the body of the 071 as there ain't much room left with the athearn chassis. It also comes as a wires only decoder and its what I would recommend for your SD9 and SD45.
Here's the link
http://www.dccsupplies.com/shop/product_info.php?cPath=459_182_394_212&products_id=2861
The first of the buildings has arrived from Glenderg for the main Street of Ballykay and I'm well impressed. A Big Thank You to Richie for all his hard work
It takes 4m/12' of track, is 1m/ 3' long and 30cm/12" wide. It can hold 12 x 201's, 16 x 071's or 20 x 141/181's. Its the Beast of Ballykay. A big thank you to Glenderg for building this monster shed for me
I Can't understand why the RPSI won't take a rake for there will come a day when 201's and 071's will be preserved and will need something nice to pull behind them besides those God awful green Mk's