Only speculation there, but

• multiple posters insist that a combination of extreme cold and misplaced electronics is enough for condensed, frozen and re-melted water to get to unprotected places (nothing complicated with the ventillation and not even snow is needed), it happened in the past in alpine tunnels;

• the potential responsibility of Eurotunnel (in cooling the tunnels) came up, but it has been pointed out that Eurotunnel has seawater cooling;

• one precedent for failures in extreme cold after more than a decaded of service was mentioned: out of a series of 26 Swiss locos built in 1946-48, 20 broke down in the winter of 1962/3, due to blown snow getting into places it shouldn't.