Jonathon: Thanks for the response. I like the idea of adding the Advance Approach to the mix. But I don't think this would apply to all railroads, past or present. So, you might want to make it optional.
As far as your question about the signal indications before a turnout, I know that the Union Pacific uses two heads at these locations. The upper head indicates the state of the Main route while the lower head indicates the state of the Diverging route. As long as the turnout is aligned for the Main (Normal) position, the upper signal is relevant while the lower head is either Red or blank. Then, when the switch is thrown, the upper head shows Red and the lower head shows the ABS indication for the Diverging route.
There are corresponding signal aspects for when the turnout is reversed and the train is headed for the diverging route or siding. Red over Green is Diverging Clear. Red over Yellow is Diverging Approach. Red over Flashing Yellow is Diverging Advance Approach. And Red over Red indicates that the block through the turnout is occupied.
This makes sense to me. But again, not all railroads will do it this way. I have seen pictures of U.P. signals where the lower head consists of just Red and Green aspects. That might be workable for a model railroad where this is quite familiar.
