Are there not individual signalboxes for the arms of the triangle? It's been a long time since I saw a diagram of that area, but thought it was all mechanical AB.
No, and indeed there never has been.
Greenford East SB was formerly Greenford Station East SB - one of a pair of signal boxes either end of the GW station on the Old Oak - Northolt Line.
Greenford station east has almost always controlled the whole triangle, save a short period of time when Greenford East Jn had its own signalbox (Greenford East Loop SB)