Who is "I" in the opening lines of the third chapter of Book I in A Tale of Two Cities?
The "I" at the beginning of the chapter is the narrator himself, who temporarily steps in to describe the feeling of being so utterly separate and so utterly mysterious from the humans that we are perhaps sitting just next to. If the first paragraph of this chapter is read, the reader can clearly see the purpose of the narrator in suddenly bringing his own perspective into the story:
A wonderful fact to reflect upon, that every human creature is constituted to be that profound secret and mystery to every other. A solemn consideration, when I enter a great city by night, that every one of those darkly clustered houses encloses its own secret; that every room in every one of them encloses its own secret; that every beating heart in the hundreds of thousands of breasts there, is, in some of its imaginings, a secret to the heart nearest it!
By suddenly bringing in his own feelings, the narrator helps his point to be understood. He brings in a personal experience of his own feelings of mystery when he contemplates the other humans around him to better prepare us for the way in which the three passengers in the mail coach are utter mysteries to each other, each with their own purpose for travelling and their own lives that are secret from the other travellers. This of course heightens the mystery of the scene and builds suspense in the reader as the reader wonders what the "mysteries" of these passengers are and how they will impinge on the narrative.