Making Uncoupler tracks work Lionel Fastrack

Yep.  My experience with operating couplers on rolling stock in 3-rail O gauge is like yours, hit or miss.  And more often than not, a miss.  (Remote electrocouplers on command-controlled locos are pretty good though.)  I've found that if you slowly pull the train out of the siding and catch the coupler while the train is on the move, it's more apt to open than it is when sitting still.

The layout we had growing up was designed so that all sidings were near the edge of the board.  So if the remote uncoupling track didn't work, you could push the tab yourself, or lift the car by hand, and disengage the coupler from that of the other car.  Yes, that kind of spoils the illusion!  But with O27 curves, we weren't about to convert our rolling stock to body-mounted Kadee couplers.  And with all of the different brands and eras, there are too many variations in geometry, design, quality control, etc.  So the "Hand of God" was the only solution that worked 100% of the time.

@Ted S posted:

...I've found that if you slowly pull the train out of the siding and catch the coupler while the train is on the move, it's more apt to open than it is when sitting still.

... the "Hand of God" was the only solution that worked 100% of the time..

+1 to Ted's response.

Years and years ago I helped my grandfather rewire his uncoupling tracks so that they were running on a separate accessory transformer, NOT on the track power. He had it juiced up to like 18 volts and the uncouplers would usually just SNAP open! BUT and this is a warning - you MUST be quick with your uncoupler button application. You must get on and then off  that button quickly. The uncoupling coils will heat up quickly at 18 volts (I think they are designed for 12 volts) one second (count in your head, "one-one thousand") should be the longest press. I frustratingly held an uncoupler button for 7~8 seconds trying to get the coupler over the small coil and I smoked it. Dead... Another time I repeatedly pressed the uncoupler 10~12 times in only a couple minutes trying to get a train separated and there was no 'magic-smoke' - but still, I had killed the coil with heat buildup.

Also, I have recently had some luck cleaning and lubricating the uncoupling plates/bars/pins/knuckles with mineral spirits and small wire brushes and/or qtips and then applying Labelle 113 oil- lots of places to buy it online and local l hobby shops usually have it too. Good Luck!

Labelle 113 coupler oil

A properly rated PTC in series with the coupler will prevent you from cooking the coil, and it'll automatically reset after it cools down.