Invisible Signals are signals which are not displayed on the dispatch operator’s desk. They are typically used for automatic signaling, which requires no action from the dispatcher to get trains through. Furthermore, invisible switching signals can be used to mark switching boundaries in railway stations (“no switching beyond this point”)
Invisible Signals can be of any type, but signals with a trigger attached cannot be of the invisible type (i.e. the state of the signal is not displayed, so it acts like an invisible signal in that regard).
Invisible Signals have their own object type. Visible Signals are in fact extensions of invisible signals, the extensions being for the purpose of the symbols showing up on the scene diagram. Invisible signals can made visible by converting them – all you need to do is select the type (reflecting the orientation of the signal) and the location where you want the signal to be displayed.
Train Dispatcher Note:
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TDP uses the term “Hidden Signal” which are automatically “placed” at the end of regular blocks which are not next to an exit off the territory nor have a regular signal. Unless there is an exit, you will have a signal, either a visible one created by yourself, or an hidden one. As far as we know, the hidden signals are a runtime function of TDP running, not based on information in the track layout file except for the absence of any signal in these situations.