To take the example of why London to Berlin by train is viable, and NYC to Chicago is not take a look at a map. There simply aren't many places between NYC and Chicago (except maybe Cleveland), while London to Berlin crosses whole other countries, many major cities, and almost no empty space except for under the channel, despite being slightly shorter.
And they chose that example because it's a good one for their case. Something like NYC to LA obviously would be much less so.