Hi Thor

When does a green light change to red? Answer: when any wheel set enters the next detection zone which is located a few ten meters beyond the signal

When the loco has passed the light mast or when the whole train has passed the light? Answer: The person making the decision to obey the signal ("the train operator") is always located in front of the train. So the signal turns to red when a few rail vehicles have passed the signal by.

Or is it when loco touch the block or train fully in block? Answer: There is no concept of a train being fully in a block (a train may occupy more than one block). So according to your definition, when the train has left the previous detection zone (ending with the signal) making the previous signal turn to yellow, green yellow green) depending on the rules.

Cheers
Jean

Hi Thor
Real train signalling
Being a Frenchman, living in Paris, signalling is like this:
- double red when a train just passed the signal,
- one red after the train has cleared the first detection zone (we call them "Cantons, 600 m lengths) after the signal and entered the next "canton"
- one orange after the train has cleared the second "canton" after the signal and entered the third "canton"
- one green after the train has cleared the third "canton" after the signal and entered the fourth "canton" just preceeding the next signal.

Detection is made by creating a voltage difference between the 2 rails and when one axle shorts both rails it triggers a relay of the "canton"
This is for main lines with normal to heavy traffic.
This is a very simplified description of French signalling but I doubt your are interested in more details.
The fun is that here signalling is affected by directions the train takes and the speed limits they have (30 km/h, 60 km/h, .., 200 km/h
TGVs because of their speed have signalling in the cabin (too fast to see lineside signal when it rains or snows at 320 km/h)
Each country has its own set of rules but in continental Europe they tend to be standardized for obvious reasons

CS3 signalling
- after a signal you have a detection zone (1 to 2 m), usually solved by isolating one rail and connecting to one S88 entry
- On the CS3, one event triggered by the occupation on this detection zone turns the signal to red and when the occupation is finished, turns the signal into yellow or green depending on the complexity of your signalling.
- in your case (you make your own luminous signals) you need a M84 (60842) to switch the red or the green LED.

On my layout, I use Rocrail with a CS3 and the detection zones are different because the signals are cosmetic and don't stop trains. Digital commands stop them.
- so to make matters simple, a zone before a signal has 2 detections. 1 during the last 20 cms before the signal to stop the train if the signal is red and the rest after the previous signal slows the train to its station speed when the signal is red.

I could continue for long but you wouldn't be interested
Here is a video of my layout with mechanical signalling

Of course using train software like Rocrail, you may have complex signals indicating:
- speed restrictions because of deviating turnouts
- advance signal position
- departure signal (the signal is green but another one gives the order to leave when the schedule is met.
- humping yard signals

Cheers
Jean