hehe ok, thx for the tips 
so, i have used the same technique, one wagon for fx calcul, and after would like to apply the same motion to cloned wagons.
but! there s a problem, who is well explain here http://www.glenbrook.k12.il.us/gbssci/phys/projects/frig/yepbyrji/coaster.html , as wagons never have the same speed, and as they dont be attached, we can see space between them increase and decrease… as wagons have acceleration/deceleration.
what is the offset you speak about?
for me i try to apply the motion with just shift frame… and that dont work… and its normal… 
but maybe that work with yours because you dont have too much acceleration during the track?
anyway, yours works really good. keep it up!
ps: the bones technic to have good bank angle is very good, thx for the tips 