Which specification though? At the time they were being designed and procured, I think the specified tolerance for frequency change rate on the grid was 0.125Hz/s, but this was relaxed in 2014 to 1Hz/s (see https://www.nationalgrideso.com/document/10771/download). How much the effects of relaxing this on existing equipment was considered, it's not clear.

Similarly, the grid's license gives them a ±1% variation around 50Hz (so ±0.5Hz from https://www.nationalgrideso.com/balancing-services/frequency...).

So we have a power anomaly where the frequency moved >0.125Hz/s and dropped well below the 49.5Hz license threshold. I don't know what the procurement spec was, but I'd be very surprised if this event was "in-spec" for the trains, so in the absence if this it's probably reasonable to say that GP is incorrect.

Transformers get quite unhappily melty if you drop the frequency too much, so if your cooling system isn't designed to cope with this, it's probably better to shut down than have an exciting fire.

The grid's license only requires them to be within ±0.5Hz of the nominal 50Hz for at least 95% of the time. Anyone designing a system which needs to operate reliably ideally ought to design it to operate over the full 47.5-52Hz range, or at least not fail so completely it cannot be restarted. Low frequency demand disconnect only starts at 48.8 Hz, ao any system that can't handle that is effectively setting itself up to be the first thing to fail when there's a power shortfall. This is utterly inexcusable for a train given the amount of disruption caused by multiple lines being clogged by failed trains in multiple places.

I don't disagree that a train that can tolerate a wider range is desirable, but lots of things are desirable. It's also desirable to give everyone their own carriage, or have a free bar on every train. But at what cost?

* Wider tolerance means heavier electrical and cooling systems, increasing both capital outlay and operating cost for both track wear and maintenance.

* The trains procured were essentially "off the shelf" from Siemens and would have been designed and manufactured for other operators before the DfT put the tender together. A wider operating tolerance would have therefore either ruled out the Siemens trains, or at least significantly upped the cost as they would have had to redesign them.

An annual season ticket from St Albans to London is about £3,600. How much more should that passenger (as well as the taxpayer) be paying so that they can travel on trains that work when this type of event happens? How often does this type of event happen?

> This is utterly inexcusable for a train given the amount of disruption caused by multiple lines being clogged by failed trains in multiple places.

People being stuck on trains for a few hours is pretty far down the severity list of "things that could go wrong" when the power goes out. That said, what I do think the outcome will be here is they'll figure out if/how the driver can self-reset this type of issue rather than requiring a fitter to travel out.