Just played a Comp Stadium match on Powder Keg Mine where we noticed that when there are 6 team mates on cart (full team plus Kiri Clone) the cart stops. It will then continue once there are no longer 6 in range.
Have not been able to test this properly as trying to wrangle bots to sit on the payload is worse than herding cats.
WNHFJJ is the code. It does not seem to happen at that same point with fewer than 6 on.