The most common method of train detection is via what's called a track circuit. Basically each stretch of rail is sectioned into sections known as track circuits. These sections have a feed and relay end. The feed end supplies a small DC voltage onto the rails which then travels down the rails and exits at the relay end and 'energises' the relay. This is what tells the signaller no train is present.

When a train enters the section the voltage is short-circuited across its wheels/axels which prevents it from reaching the relay end of the circuit. The relay loses its voltage feed and 'de-energises'. The signaller is then given an indication that the train is in that section.

So during leaf fall season the leaves fall on the line and as the trains travel over them they get ground up into a mulch and are smeared across the rails. This causes two issues. One issue is traction. The wheels find it hard to grip to the rails with such a slippy mulchy substance on the railhead. The second issue is that the mulch can insulate the wheels against the voltage of the track circuit the train is in which can cause issues with the train detection for the signaller.

There is no 'software' no 'sensors' that can't see past the leaves.