Just replaced the rapido couplers equiped trucks on one of my N scale boxcars with 2 M.T. Bettendorf trucks. The rest of my locos and rolling stock use Accumate couplers. The M.T. trucks equiped car , specially at low speed , never keeps the same distance with respect to the preceeding car. It seems like there is an oscillation . That's what I call the accordion effect. Any explaination ?