I'm not sure how many people realize that hearing one half of a phone conversation, no matter what vocal volume, is incredibly distracting and annoying. It is impossible to tune it out, because one is hearing disjointed, unconnected things, and the ears just automatically switch over to this stimulus every time it presents itself. At home, one can choose to exit the room. If you're stuck on a train, you don't have that option. "Catching up" with your friends, which is apparently what the letter-writer does for long periods of time while travelling by train, is an unwelcome distraction to the people who are basically a captive audience for your personal life.
(It's nowhere near as irksome when you can hear both sides of the conversation, although that too can be annoying when people discuss their Scrabble habits in explicit detail without bothering to lower their voices... but, strangely enough, such things are much easier to ignore than if one is only hearing one side of the conversation.)