If only it were that simple. Just because a train is fitted it doesn't necessarily mean that the VOBCs (Vehicle Onboard Computers) are functioning - if they're not both available then a train won't be going into the CBTC area. Even since 21551/2 has been stood down trains that cannot run to Hammersmith have been presented at Edgware Road. This will remain a problem pretty much until there is no tripcock railway left. Additionally, the S8s have always been the bigger priority for SMA1/2. All S8s must be VOBC equipped before that section can go live; unfitted trains will not be able to proceed south of Wembley Park, which significantly hampers service options. If a train without a VOBC does get to Finchley Road, you're looking at a significant suspension whilst the train is worked forwards in RM until it is able to reverse (not necessarily at Marlborough Road either...). It has always been possible to operate a fully service in SMA0.5-2 with less than all of the S7s converted (I forget the exact amount) - they'd just be kept to District Main services.