Newman Atkinson says:

Hi Paul, Depending on your couplers, is what you can use. If you are using the old horn couplers then there is a uncoupling device that you can get that sits in between the rails that the bottom of the coupler slides through and if you pull through forward it will hopefully stay connected. If you stop or back that coupler in to the device it is suppose to separate and off you go. It was never very reliable and depending on the coupler’s condition they would many times cause unwanted uncouplings. This type was usually used on a siding where a car would be delivered.
For the knuckle couplers there is usually a magnet that is set under the track and it also is suppose to stay coupled when pulling forward but more often than not would divide your train. Some people would build a mechanism to raise the magnet up to the track when uncoupling was needed. They work OK but to build the mechanisms to raise and lower is somewhat time consuming .
When using the knuckle couplers that are all plastic the hose piece under the coupler is usually plastic and if that is the case then a mechanical or magnetic devise will not work.
Myself and most any club or personal layout that we have actual operation sessions, even with the metal hose trip pins undder the coupler, the best of them use a good pointed pencil, or an official uncoupling tool from the hobby shop and point them down between the cars and into the top of the two couplers and that will spread them to separate. But that is what the most of us do. I also use a magnet that I have a magnet on each side and drop it down over the coupler so the magnet is on each side of the coupler and it willpull the hose uncoupling pins apart there-by opening the coupler.. This works fine except if you have diaphrams on your passenger cars and cannot get the magnet or pencil tool down to the coupler. It is difficult to uncouple. When using these types of couplers to back up a car for spotting the car is uncoupled and the coupler is pulled to the side and then they back the car with it not coupled to it’s spot and it will stay where you put it.
On many of my long cars where they will raise and lower going over uneven tracks and slip out of each other, I use the Kadee no. 118 couplers that are real working double shelf couplers They are very difficult to uncouple without a hand tool or a magnet. I have used them on my superliners and with the diaphrams on them They are near to impossible to uncouple. They are best to roll them over a fixed in track magnet on a side track to uncouple these. But to really answer your question the true model railroaders use a simple pencil or hand uncoupling tool and even a modeling fine point paint applicator to release the couplers. Their rule when doing operations is not to touch or lift the cars to uncouple them during the operations to run them as real as possible. Go get a took from KADEE Couplers or just a simple sharp Pencil. They work much better. Hope I have helped. from Newman