KaDee or not KaDee. That is the
  question. The original Marklin couplers
  (on the right) are big, clunky and keep
  the cars too far apart. They can also be
  hard to couple on curves. And the
  uncouplers are visible mechanical things
  made of wire that can uncouple a train
  that jerks a bit going forward across it.
  Micro-Trains, on the other hand, makes
  a KaDee style replacement coupler that
  looks great, works well, keeps the cars
  scale close and uncouples with magnets
  -- an electromagnet under the table can
  be energized only when you want to
uncouple something. But that's not the whole story. KaDees are so small that uneven track or roughness
enough to bounce a car can cause random decoupling and tight durves can drag your cars off the track. The
bulky Marklins are more forgiving in this respect. So there's a trade-off between scale and less troublesome
operation. Obviously, a carefully built layout with bigger curves would make KaDee the right choice, but the
beauty of Z is the ability to build an empire on a coffee table.

       Here I'm showing
  some very common
  track configurations
  on an interesting small
  layout (mine). The
  Kadee style couplers
  were shot on the track
  in the left photo and are
  shown on the left. Same
  for Marklins on the
  right. I used reverse
  track configurations
  because the two styles
  of coupler open on
  opposite sides. The
  upper photo on each
  side shows the coupler
  center position, while
  the lower photo shows
  the strain on each
  coupler type going
  through that section
  of track. It's obvious
  the Marklins do better
  in each case.

     An alternate solution is the truck-
mounted coupler. This in NOT scale
as real railroads mount their couplers
on the cars. But they also have room
to make much wider curves. To the
right you can see that the coupler on
the black tender turns to match the
track direction while the one on the
brown flatcar does not. Micro-Trains
makes trucks with KaDee-style mounted couplers. To install them you at least have to remove the body-
mounted couplers that come with the cars. You may also have to cut out the boxes that held the couplers
in place to get clearance. But it can be done, looks more real than the Marklins and runs well on most layouts.
Due to the mechanics -- three pull-points between each car instead of just one -- your trains can't be as long,
but then there's no room for a long train on a small layout. So it can be an acceptable solution.
     A final consideration is weight. Z trains are so small that plastic only cars in long trains tend to pull off the
rails on corners. So all of them are either cast metal or weighted. This makes it harder for the equally light
engines to pull trains up hill. My 4-6-0, for example, has trouble with only 3 cars on the loop up to the bridge
on my layout. Removing the weights allows me to pull the whole passenger set but increases derailings. If I
figure that one out, I'll let you know.
     In closing, here's my rolling stock: