I am probably not the first, but I have had an entertaining evening pressing the decouple button on London Commuter trains when I shouldn't. The results are quite interesting as it seems that in some circumstances the train sets are "Quantum entangled". If you let the front half go off on its merry way the back half is often still drivable - until the front half either goes through a red signal or hits the buffers at Victoria or Brighton, at which point the back half (which may be many miles away) suffers the same error. And the fun ends. Similarly if you are on an AI train, decouple, let the front go off and the try to take control of the back half the front half - regardless of how far ahead it is - stops immediately. Other results are less interesting. Decoupling an AI train in a station means that it never leaves, and arriving in a station with only half a train means that either your arrival is not recognised or you get an unexpected formation error. Attempting to leave a station with half a train often results in the red lights refusing to change. Still - kept my busy for a whole evening trying out different scenarios.
Try this, just out of curiosity: decouple, then reset the train length indicator from 8 to 4 and see if that “releases” the front half
It ought to be, yet I found that, weirdly, resetting it after coupling two trains was what caused the PIS to change to the correct destination
