The first uncouple project used a servo to raise the coupler box and it was very reliable. However, I wanted one that would work on a verity of cars other than box cars (it required space) and be more prototypical. That's when I decided on using a servo to pull a string attached to the coupler knuckle. We have had several operating sessions with two cars that have DCC controlled couplers. Generally, uncoupling is reliable providing there is slack between the couplers when attempting uncoupling.

By consisting the uncoupler decoder in the car to the engine, it is possible to have the uncoupler sound in the engine activate when the car uncouples.

To help achieve reliable operation, all wheels have electrical pickup. It is important to have good electrical contact and still have free turning wheels. One way to do this is to put a washer (NWSL 1106-4 for HO) between the truck and the wheel. This works like a loose slip ring. Pairs of washers are connected together by soldering a piece of small wire between them. Washers require the correct type of wheels. The needle part of the axle and the wheel must be metal. Walthers HO wheels 933-870 and trucks 933-1008 are examples.

Below are some pictures showing a car with an uncoupler scheme using a custom decoder and a small servo.
Click on picture to enlarge.

A spring wire is connected to the servo arm to increase the effective movement of the arm and allow for tension on the coupler arm without breaking something.
Nylon filament string is used to connect the spring wire to coupler arm.
The string has a reasonable straight run to the coupler.
Coupler closed.
Coupler open.
Both trucks have electrical pickup (only 1 shown here). The small wire that electrically connects the washers to the feed wire can be located on the top or on the bottom (as shown) of the truck.
The small bronze washer is just visible behind the truck bearing.