In any case, the problem in this particular case appears to have been a combination of cold temperatures and accumulated powder snow. Afaik, Eurostar trains are based on air-cooled asynchronous electric motors, the power electronics and transformers are probably cooled via a water or oil circuit and radiators. Not sure about that, though. Either way, the systems are presumably fully sealed to prevent rain or melt water from ever making contact with the live components. Rapidly melting snow could cause mechanical damage as blocks of it fall off at 100mph. It’s not clear if a previously inactive (i.e. cold) traction system was switched on as the trains entered the tunnel.

In addition, there are batteries on board for ride-through and hotel loads. If the terminals on those were cold when the trains entered the tunnel, it’s possible the high humidity there caused condensation to form, shorting out the subsystem and possibly, the entire train and OCS.

In any event, the only way to get a handle on these gremlins is to test the system under controlled conditions, i.e. in a climatic wind tunnel. That’s not something Eurostar can do ad-hoc, though.

Their biggest failure by far has been grossly inadequate preparation for efficiently informing, supplying and then evacuating thousands stranded passengers in the event of a breakdown inside the tunnel. Ensuring adequate reliability engineering and disaster recovery planning happens is the responsibility of senior management, pure and simple. Operations staff clearly didn’t know what to do. If you’re going to run trains at 15 minute intervals during the Christmas period, you need to anticipate that you’re going to have 4-6 trains in the two tunnel tubes at the same time. It didn’t help that all communication between management and drivers had to go through Eurotunnel while the trains were stuck, but mostly it appears to have been a case of Eurostar simply not having its own ducks in a row.

They did think about it. a vanishingly small percentage of their annual ridership was inconvenienced by the extraodinarily rare break down. Transferring some of the passengers to another train and using rescue locomotives to haul the rest out, worked.

…. why don’t Amtrak trains in California carry around two cars full of batteries for when the locomotive breaks down? The wheels on diesel trains are driven by electric motors. If they dragged around two cars of batteries all the time, when the generator or engine on the locomotive broke down they could switch to battery power and get the passengers to the next station. Shouldn’t cost more than 10, 15, 20 million dollars a train…..

Fortunately, people like you will not be in charge of determining anything related to HSR, so I’m not terribly worried.