The black boxes are not geographically accurate as I understand it. There is however a separate one for each direction for each platform, and having watched it for a few cycles of the timetable the southbound Chiltern stopping service always appears in the right hand box next to 7122, with its departure time in the other box to its left. I'm fairly sure the train will however be physically alongside P4 during its period between arrival and departure? Next one at 1300, 1U27, appeared in the LH box on arrival and then became 1H46 in the right hand box.
Likewise in the down direction, the box for 7129, is not drawn alongside the platform where a train would be standing.
It's not always the case with those maps, but they are pretty accurate for the Banbury area. The only mistake is showing a full length platform 4, as it is only half the length of platform 3 and located at the northern end. The starting signal (OL7122) is actually located alongside OL7120 and OL3124 so is some way from the end of the platform - something drivers are not particularly happy about and are pressing for an 'OFF' indicator so they don't misread or read across their signal. So, yes, they would be located alongside the platform.
In the down direction, OL7129 is actually located where it shows on the map. There is a banner repeater for it at the northern end of platform 1, near the North Box. This is to allow a freight train to be put recessed there if necessary and the Down Goods Loop towards OL7143 is fully occupied.
As for blank signals, perhaps 'poggs' can enlighten us, but I suspect that the data feed is missing, or incorrectly interpreted. There are other black signals in the area that was resignalled.
GCX is bidirectional through the station, but in the peak, it would delay incoming services, so doesn't tend to get used as compounding the delays results
Having thought about what you said, I think you'll find Gerrards Cross is only bi-directional in the up direction, so
outgoing trains would be delayed when it's used and there's no way of one train passing another in the down direction - so that is only possible at South Ruislip, Princes Risborough (slow linespeeds) and Bicester North (blocks the up line). For me, that's not enough.
High Wycombe in the up direction does indeed have the odd timetabled pass move, but only during quiet times of the day as there's nearly always a down train on the horizon that would get clobbered, especially when it comes to a delay recovery scenario. That's not so much of a problem though as you have the passing point just down the line at Princes Risborough.
Also, there really could do with being a couple more signals between Haddenham and Bicester too as that can also hold things up a little.
