Perpetual Night

A similar fog to the one depicted in this episode occurred in 1952, shrouding the city in a dense mist and killing an estimated 12,000 citizens. The deadly ingredient of the fog came from emissions from coal burning for residential use, power plants, and other means: sulfur dioxide was turned into sulfuric acid by an unfortunate combination of physics and chemistry, essentially turning the fog into a huge acid cloud which was inhaled by the Londoners over the course of 5 days.

Cunard line boats (such as the RMS Umbria) coming from New York would not go directly to London, but to Liverpool. From there, it would be much faster to board a train than another boat.