This is a photo of the installation of an A1X (now the A4X)decoder in an Athearn Genesis F3A.  The installation is very simple. The decoder installation in a B-unit is the same except no lighting to deal with.  Athearn wire clips do not fit your board ( or probably anyone else's). Therefore I wound up soldering the leads to the decoder board. Since you have to remove the Athearn board I guessed properly that one needed  to add a resistor. Unfortunately I found that 560 ohm was not enough. So I installed a Miniatronics 1.5v bulb with a 620ohm resistor and  it works fine.