As I've explained before - how would a rescue loco actually get to a failure? Given the intensity of service, there will always be at least a couple of trains between the rescue loco and the failure.
Better to rescue the failure with something exactly the same, ie the train behind.
There are technical pros and cons to rescuing with a locomotive, some of which I've already touched on - the cons include the possibility that the fault may still transfer, especially a brake fault, and in this case it may only be able to be overcome by attaching more vehicles than just one locomotive, anyway. Also, diesel locos might be more resilient to power failures, but I can't see any off-the-shelf options being routinely used in confined spaces.
However, a dedicated loco able to be resourced for rescue purposes might have been
quicker - often the assisting unit must de-train, which takes time when you have a long and busy train full of disgruntled passengers, and we must not forget that overnight failures in service do not always immediately result in a long queue of trains - this failure occurred well before 5am, with relatively little traffic around, yet it took a couple of hours before the assisting unit was required, prepared, moved forward, coupled up, tried to move and then failed too.
The train I was on this morning (advertised as fast to St Pancras but actually stopped everywhere) got stuck alongside the sidings at Cricklewood at about 0940 for 15 minutes or so while this crossed into the depot in front of us. And when we eventually reached St Pancras at 1010 there were still no southbound trains. Neither of which makes particular sense based on the above.
A "split service" was implemented to run either side of the Thames after about 0730, with trains only being reintroduced to the Core once it started to reopen. That being said, split service implementation is often messy and basically theoretical, as there are so many units still trying to cross London or stuck in queues/awaiting crew/with major delays some way outside the zones. It went a bit better here, though not much more so, simply because the peak arrivals were still ramping up to full capacity when the decision to pull more of the service was made. Or such was my observation of it.
Let me introduce the team to the amazing concept of emergency crossovers. And managements that know how to spell "resilience".
The Core has bi-directional signalling and emergency crossovers, but one of my biggest bugbears is that they take absolutely ages to start to be used, and when they do, somehow what should be a passage of a few minutes across a small part of London turns into a half-hour detour with numerous opportunities for head-scratching and wondering what's going on. In fairness, this also applies with the bi-di running on the Brighton Mainline to a much greater extent, though it seems to have been ironed out quite well on some of the longer-distance mainlines, which is curious (it's even used to overtake trains to get expresses ahead of delayed stoppers on the GEML, for example, which is pure fantasy on the BML!).
