You may want to reread the previous drabble before this one, as they are sequenced.
It's not a long train ride, in perspective. Watson has taken trains to the Continent upon many occasions, not least the most memorable of which was his journey to Switzerland, where he lost Holmes for the first time.

Still, the ride from London to Sussex seems unbearably long. Watson feels himself in limbo. He knows that he has made a choice, and yet, perhaps it's all a dreamy mistake.

Forgive me, he imagines the chugging wheels of the train to be saying. Forgive me, forgive me, forgive me.

He can't be certain to whom he is repenting, God or Holmes.