The second example is quite good imo. You would never say “I’m going to a bathroom.” Even if you were in a stadium with hundreds of bathrooms, you would still say you are going to the bathroom.

you are justifying this statement “because of reasons” :) this is not a logical explanation. there are a bunch of bathrooms in the world. Am I going to the one specific that was mentioned before in the context? then it’s “to the bathroom”. is this bathroom just a random one that I will encounter on my way? then why not “to a bathroom”?

same with trains: even if you are speaking about a train route with a specific number, you are not talking about a specific train, then it should be “I’ll be taking a train”. if you are going to take the specific locomotive for a ride in a museum, then, obviously, you are going to take “the train”. this sounds logical for the usage of Articles in the English language, at least in my head.

same with doctors: if you are going to a specific doctor, and your opponent knows which one by the context, then it should be “to the doctor”, otherwise it would be a random doctor that will be assigned to you as soon as you arrive to the clinic, it cannot be “the doctor” by the same principle I always thought exists :)

whoa, another meaningless “the” appeared. “The English Language” phrasing is used despite there are at least four? five? English languages.

so… it’s complicated :D but this does not prevent English from being The Language of the World, and I cannot name a language that could have been on its place.