On every Minitrix loco that I have worked on, there seems to have been an unwritten rule.  The right hand side of the loco is positive.  The left hand side is negative and is connected to the chassis.  With this arrangement, if positive power is applied to the right hand side of the loco, it moves forward.

Not with this loco.  It runs backwards.  Normally this means that there is something that is wrong in the assembly - the motor is upside down, the axles are put in the wrong way, the contact plate is reversed.  But not so with this model.  You can't put the motor upside down - well you can, but it won't work.  If the contact plate is reversed, the circuit board contacts don't meet up with the motor contacts.  The same thing happens if you turn the motor round.  The only way I could get this loco to run forwards if the right hand rail is positive, is if the copper electrical pickups are on the left.  This makes the chassis positive, rather than negative.  It's not a problem, you just put what I would consider the front bogey at the back and the back bogey at the front.  Problem solved.

But it breaks what I always considered as a hard and fast rule.

Front is to the left, the motor brushes are at the rear, the gear cover screws are facing us.  The bogeys have their electrical pickups on the near side (ie left) of the loco (and if you didn't read the bit above, this is unusual).

Place the plastic gears into position in the slot in the top of the bogey