As far as I am concerned the best way to do this is to run the uncoupling tracks on fixed voltage and put an undersized resettable fuse in the power feed to the uncouplers. I posted detail on this some time ago.
The principle of operation is simple. You size the resettable fuse so it will definitely trip a few seconds after the uncoupler is energized. If you release the button before that time the fuse never trips. If you hold the button too long the fuse trips, protecting the uncoupler. The fuse then resets a few seconds after the button is released. If the button is never released the fuse never resets. No timers, no relays.
The Fastrack uncoupler draws about 1.8A at 14 volts. If you use a resettable fuse that trips at, say, 1 ampere you will get probably 2-3 seconds of actuation. I use a 1.5A trip fuse, that gives me about 15 seconds. You will need to choose the exact fuse based the voltage you use to run your uncouplers and the operation time you want. The fuses are cheap, pick up an assortment in the .8 to 1.5 amp trip range or thereabouts and experiment with them.
Note that operating cars use much more current than the uncouplers. If you are using operating tracks this solution may not work for you.
The same solution could also protect switch machines, although I don't know what size fuse would be appropriate.