Straightforward? As RAIB▸ say, they will need to look at:
1. the actions of the people involved
2. Chiltern Railways’ arrangements regarding the training, competence and fitness of its train drivers
3. Chiltern Railways’ management of risk associated with its drivers operating on the London Underground network
4. how risk associated with signals passed at danger by non-London Underground trains is managed on the London Underground network
5. any relevant underlying factors
As is often the case with incidents which RAIB investigates, the direct cause may be straightforward, but the underlying causes and contributory factors may not.
Point taken. Though that's straightforward compared with a scenario where the incident could happen
without rules and procedures being broken.
It'll be interesting to see the conclusions.