The plug is integrated into the decoder

This small 6 pin interface is used mostly for N scale locomotives due to their small size.

The locomotive side is a 1x6 female socket with 0.05" pitch.
The pin out is as follows:

Pin 1 Motor Right

Pin 2 Motor Left

Pin 3 Right Rail

Pin 4 Left Rail

Pin 5 Front Headlight (F0F)

Pin 6 Rear Headlight (F0R)