DCC in N&W J4-8-4

[Bazz]

I am new to DCC and recently purchased a 'N' gauge Bachmann N &W J class 4-8-4 DCC ready. When I opened up the tender to see what sort of decoder required I found a board already fitted where I only expected to see a plug. What is the board? and do I need to fit a decoder and if so what kind. Is the board as fitted designed to run on DC or AC or both.

[r0bert]

http://www.bachmanntrains.com/home-usa/board/index.php/topic,1182.0.html