Bob Roth says:

Uncoupling of Kadee couplers:

Number 1: Check the height of the couplers. Kadee has a coupler height gauge and you need to use one to ensure the couplers are all properly adjusted. The coupler height gauge is a very important tool for this particular task. The coupler knuckles all need to be the same height so they mate properly and the trip pin on the bottom of the coupler needs to be high enough to clear the track when the equipment goes thru turn-outs or over cross-overs or grade crossings. Kadee makes a wide variety of couplers with different shanks, knuckle heights and spring assemblies to fit most pieces of model railroad equipment.

Number 2: Check your trackwork. Check the spots where the cars uncouple. Is the track level? Does the track dip or sink when a truck (set of wheels on a car) passes over a given spot? Watch the couplers (from the side) in these areas as they should stay level with each other; there may be some up-and-down play in the couplers, but too much vertical play and they will uncouple. Sudden changes in elevation can be a particular problem, especially with longer cars where the trucks are set back from the end of the cars.

Number 3: Check the knuckle of the coupler itself. Ensure the tiny spring which holds the coupler knuckle closed is in place. Without this spring the coupler can pop open at any time.

Model railroad equipment with properly adjusted couplers can be a joy to operate, especially with (hands-free) switching operations using the magnamatic action of the Kadee couplers.