Bachmann locomotives come with a 'light board' of some kind. They have to be removed, or wired around, when installing decoders. This is either a light board or perhaps an N Scale decoder...but I don't think it's a decoder.
it does seem to be a decoder, can't get a decent magnification to tell whose it is though ...
blue is power for the lights, white and yellow are function negative for the lights ..
jmri would give you a closer idea ..
I think wvgca is right. The color code of the wires gives it away. I converted an older one of these to DCC and it didn't have a light board. The headlight drew power directly from the track through the front wheel set. Light boards are not that complex. They're usually just a thin board with a couple of diodes on it.