From the sound of what's been said on this thread and elsewhere if they were to ground trains they'd need to ground every single train which uses the twin pipe brake system. Which I think is basically everything on the entire network. It doesn't appear that this was down to an issue specifically with the new rolling stock. The fact that it is still in service and not grounded should be the big clue to that. The railway doesn't take chances, indeed it's perhaps the most risk averse mode of transport going in this country!
I think that, generally-speaking, (Western) aviation will trump rail for risk aversion and a focus on what they refer to as 'Crew Resource Management' - ie making sure that flight deck superiority can be challenged and that 'blame' is replaced with 'learn from your human mistakes'.
It's quite right to draw the distinction between what the RAIB can do, and what the CAA, EASA, or FAA (for example) can do.
However, to go back to the aviation comparison, time and time again, post-event investigation concludes that many disasters have occurred when more than one circumstance that would, on its own, have been recoverable-from, has co-incided with one or more other similarly recoverable-in-isolation circumstance(s).
It looks as though this is where the 737MAX investigations will likely end up, too.
In the case of aviation, it would also be easy to look at individual circumstances in isolation, and conclude that they're a manageable risk. MCAS on the 737MAX being a case-in-point. However, if MCAS characteristics are such that, reasonably-speaking, the chance of a coincidental circumstance is unreasonably high - whether that's inconsistent training, human reflex response, adverse weather etc., etc., - then it's no use continuing to claim that MCAS is perfectly OK and that there's nothing to worry about.
In the case of the MkVs, then, it may be that (as has also been alluded-to above) the specific complexity of the rolling stock, and/or the software, and/or the training, and/or the human resource, and/or the operating conditions etc., etc., could be the real risk. If that doesn't apply to other rolling stock, or other operations, or other operators, then if a 'grounding' were to occur, I would have thought/hoped that those specifics would be taken into account and the entire network wouldn't grind to a halt.
Subjectively, though, from an industry that does portray a (well-earned) image of being safety-first, it does concern me personally that there are individuals and corporate entities who may be prone to looking too narrowly at individual circumstances, and not appreciating-fully the implications of two things going wrong at the same time, nor that the risk of that may be heightened by complexity and/or lack of resource.