Lord Grantham has invested in the Grand Trunk Railway (GTR) run by Charles Hays. Murray says that Hays had died, but doesn't mention that Hays was a victim of the Titanic in 1912 and had been sailing home with his family. GTR collapsed in 1919 and, as stated, was taken over by the Canadian Government.
A sign in Downton village points to "Ripon 9 Miles" and "Thirsk 6 Miles" in opposite directions. This places Downton to the east of the Great North Road (the modern-day A1), somewhere in the vicinity of Skipton-on-Swale or Topcliffe.
Mary says "Carson's motto is be prepared." Violet then says "I'm afraid Baden-Powell has stolen it" "Be prepared" is the Boy Scout motto and Baden-Powell the founder of Boy Scouts.
After it has been revealed that Larry Grey has drugged Tom Branson, Lord Grantham mumbles to his mother, in French, "il ne manque que ça" which Merriam-Webster idiomatically defines as "that takes the cake."
Isobel uses the English aphorism, "Much cattle, much care." "Cattle" is an old form of the word "property." So the saying is equivalent to "When one has a lot of property, one has a lot to look after and be concerned with." The implication is that one should not wish to possess more than one can reasonably expect to take care of.