Greg wrote: "Might I inquire why you want to eliminate approach?"
For those who have an interest in prototype signaling, let's take a quick trip to "Prototype Signaling Class". Those uninterested may ignore the remainder of this post.
Here's one prototype-based reason: It turns out that in early prototype signal systems, some signals could only display two aspects. In order to get a third aspect, one needed to add a second two-aspect head to the mast. The early so-called "banjo"-style signal heads, including the "Hall Disc" signal, typically could only display two aspects. See for a description of the Hall Disc signal. That page reports that Hall "Disc" signals were in use from the 1870s to the 1950s, and includes some photos of this type of signal still in-use in the 1940s.
The web site states that "Disc"-type signals were the most common signal type until the 1910s when the number of installed semaphore-type signals exceeded the number of "Disc"-type signals. In some cases, two-aspect Disc signals were replaced by two-aspect semaphore signals.
Some (later) Disc signals were able to display three aspects. I have a 1903 railroad rule book which shows aspects used in interlocking applications where one "Disc" signal had three aspects - Red ("Stop"), Green ("Caution"), and Clear ("Proceed" ).
Regards,
Billybob
