Four of the trains failed *in the tunnel* where it was "warm", not outside where it was cold.

The likely cause is a mechanical design flaw resulting in uneven thermal expansion of parts in the engines made of different materials.

There's not enough information to tell but I bet when they entered the "warm" tunnel part A expanded quickly and part B more slowly, and they got out of alignment...

Whoever designed those engines is not going to have a Merry Christmas. I predict long hours starting tomorrow morning to determine how to operate the existing trains safely as well as come up with a fix that be retrofitted quickly. We're early enough in the winter season that these weather conditions could repeat in just a few weeks.

PS. Susan's explanation is more likely. The air in the "warm" tunnel will carry a lot of moisture. It's still going to be a busy week for someone to get it fixed...