A 12-car turnback was one of my projects at NR. Both platforms are bidirectional, and a 12-car can call (perhaps with one or two doors isolated by SDO, not sure), but because the train doesn't quite fit between the signals, it can't be reversed there as desired. For some years, TOCs ran shorter 11-car formations on the terminators to overcome this limitation, and NR looked at various options to move the signals at one or both ends a short distance into the tunnels to create more standage. There were many practical problems with that approach, however (very tight clearance, safe maintenance access, lack of an approved tunnel signal product for use so close to the portal), so eventually the idea of a turnback siding was suggested by the TOC. In some respects, that's better than turning back in a platform, as the platform can be used by another train while the reversing one shunts, and the driver changes ends. While the whole procedure clearly takes longer via the siding, it was a TOC suggestion and fitted with their diagrams so was an acceptable solution for all parties.