I can't remember to which trains this applies, it may well not even be the UK, but there are some railway carriages which unintentionally block 4G and 5G signal

I can't remember to which trains this applies, it may well not even be the UK, but there are some railway carriages which unintentionally block 4G and 5G signals. When they were designed in the 1990s or so, it was a side effect of blocking UV or something like that. In that case, adding WiFi inside the train, with the antenna outside the accidental Faraday cage, is worthwhile.