word choice - Why did の disappear from 山手, but in 御茶ノ水 it's in katakana?

I realize that very likely the answer to this question is likely to be something along the lines of "that's just the way it is", but I thought it worth asking to see if there were some insights that weren't immediately apparent.

In Tokyo, the JR train line called the 山手線【やまのてせん】 seems to have dropped the の from the way it is written. It seems there used to be areas of Tokyo referred to as 山の手 , which was where the elites lived and is in opposition to the term 下町【したまち】(downtown, lower class areas). I hear the 下町 often enough in reference to areas like Asakusa, but I haven't heard anyone use 山の手. The only やまのて I know of these days is the 山手 train line.

There is also an area of town called [御茶ノ水]【おちゃのみず】, where the の is still there, but in katakana. Why not leave it in hiragana...?

So, is there any rhyme or reason to how these conventions of spelling came about? Is there a relation?

Or are they just quirks of tradition that just stand on their own without any rhyme or reason?

Answer

According to Wikipedia, the correct name of “山手線” is “やまのてせん.” In the application form of business license submitted by The National Railway (then 日本国有鉄道) to the government before the start of operation in early Meiji era, it was indicated as “山ノ手線,” and remained so until / during the World War II.

However, the National Railway (then 国鉄) started to use the name 山手線 side-by-side with the rōmaji “Yamate Line” immediately after the end of the War when they were directed by GHQ to indicate station names both with Japanese characters and the Latin alphabet. Before and around this time, the National Railway people had been using the name of “ヤマテ,” an abbreviation of ”山ノ手線” for their internal communications. They simply appropriated their password to the official indication of “山ノ手線.”

In response to the movement to make the names of stations and railway lines familiar to the public and easy for them to read, along with the introduction of the famous “Discover Japan" campaign in 1970, JR decided to place 振り仮名 to the names of all stations and railway lines, and they placed “やまのてせん” as 振り仮名 to “山手線” on March 7, 1971.

Presently, the destination of direction displays in the Yamanotesen trains is shown as “山手線” in Kanji and “YAMANOTE LINE” in rōmaji.