The NEM651 6-pin DCC plug is common in N gauge locos, or even some small OO locos, such as Bachmann's 03 Class diesel shunter.
6 pins is enough to provide pickups, motor output and 2 function outputs (normally for forward and reverse loco lighting).
However, some 6-pin decoders also provide additional function outputs that can be used for extra lighting or other
advanced features like a smoke generator, or automatic couplers. Any additional outputs require additional wires on
the decoder, or are provided as solder pads for you to add your own wires to.
However, a simple 6-pin decoder is very small, and will fit straight into the 6-pin socket provided - just remove the blanking
plate first, then slide the decoder careful on.
The orientation of the decoder is important, as it wont do anything much if you get it upside down! If the loco does
not appear to function correctly with the decoder one way around, just flip it over and try it the other way up.
Most decoders and most loco sockets have an indicator to tell you which pin is pin 1, but it isn't always clear, so you
may just need to try it out.
Don't be fooled by the blanking plate provded with the loco - to the casual eye, these blanking plates look just like
6-pin decoders vary widely in size, from the cheap and chunky Bachmann 6-pin, right down to the ultra-thin CT DCX76. There
is a good selection out there, with prices varying from £10 up to £35. You get what you pay for on the whole, and YouChoos
recommends using CT or Zimo decoders where possible, and where your budget allows.