@Webskipper:
If there isn't a NEM651 socket, you need an "ESU KATO147456" replacement board. Unfortunately, I can no longer find them online anywhere (oddly, I only need 1, and not the pair). Places where I've gotten them in the past are out and say they can't get any so they may be gone (or maybe someone else has an idea, or spares?) IF, you can find this board, a plug-in 6-pin decoder often won't work. Well, it works, but you can't get the shell back on; the pantograph gets in the way. What I've done is to use the TCS EUN651P-18 decoder with a very short harness.
http://www.tcsdcc.com/public_html/Zen/index.php?main_page=product_info&cPath=6_58&products_id=410caboose hobbies in Denver stocks them at a good price and will ship.
if you can't find the ESU board, somewhere I have some instructions on how to chop and whack the factory board and use a 6-pin decoder. TCS makes this same decoder with a short harness without the 6-pin plug
@Spiff:
I've tried that Digitrax decoder (since I can't find the ESU boards) and couldn't get it to work. It seems like it should, and i might go back to it again someday. The other issue with the Digitrax is first you have to mess with the LEDs to get them both pointing up front (without shorting), get the colors right (forward/reverse), get the LEDs lined up with the light tubes, and get it all to happen without the board lifting from the less-than optimal mounting "system" and keep the shell on with all that. It seems that once the shell's been off a few times on this model, they pop off too easily.