A train driver expects aspects to follow a logical sequence. For example, in a 4-aspect area, (s)he should expect a green to be followed by a double yellow or flashing double yellow or another green. If (s)he sees a single yellow, single flashing yellow, or red then something has happened and it is considered an adverse change of aspect.
In SimSig, if a signal is replaced in such a way as to cause a driver to "witness" an adverse change of aspect, the train will phone in and report the incident, and only continue after calling their control room. This will be the case whether the signal was replaced by the player or an equipment failure.
Last edited by GeoffM on 15/09/2016 at 03:00