However, in all seriousness, I believe one field that I'm sure would consist mostly, if not entirely, of Japanese loanwords is rail transport, considering that the Inklings' rail transport is similar to Japan's IRL, and can be considered to be what little remains of Japan's rail transport.
The problem is how we can get tons of phrases to fit in. For sure, a form of "densha" (/dɛnʃɑ/) can be used for "train," and the words for "up" and "down," "nobori" (/nɔbɔɽi/) and "kudari" (/kɯdɑɽi/) can be used for inbound and outbound trains, respectively.
For those wondering, "densha" is 「電車」, "nobori" is 「上り」, and "kudari" is 「下り」 in written Japanese.
But then, there are how announcements would fit in, specifically the words in these announcements. Inevitably the Inklings would have heard them in the raw Japanese at the early stages of civilization, but these would have been phased out by their language in the later stages. Some words might as well have been retained as so-called "railroad slang" and the like. Also, there is a standard series of announcements, but as you get farther out of town, one would hear variations of that standard series (but not in regional dialect).
I might provide some examples of such announcements in a future post so you guys can fit it all in.
Also, if they don't fit with script regulations, it may be a case of using the other glyphs we do not know about.
The IPA was me doing the nearest approximate - I did not pick up the standard vowels to use when transcribing into IPA. I only know that this and Japanese share phonology to some degree.
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EDIT: Train types can be loanworded from Japanese too.
"Local" trains (A train that stops at all stops): "Futsuu densha" or "kakueki teisha" (/fɯtsɯ:dɛnʃɑ/, /kɑkɯɛkitɛiʃɑ/) (abbr. "kakutei" 「各停」 /kɑkɯtɛi/) (In real Japanese, these are 「普通電車」 and 「各駅停車」, respectively, meaning literally "Regular train" and "stops at all stations")
"Rapid" trains (skip some stops): "kaisoku (densha)" /kɑisɔkɯ/ (/dɛnʃɑ/) (Japanese 快速, lit. "high speed")
"Special Rapid" trains (yet more stops are skipped): "tokkai (densha)" /tɔk:ɑi/ (In real Japanese, this is a contraction of "tokubetsu kaisoku" (「特別快速」, lit. "special high speed"), and is written 特快. Lit. "special high")
I don't want to be the one who dominates this thread, so...........