Who was Mr Harris in Dickens?

In Murder on the Orient Express, it’s a key plot point that Poirot has trouble getting a berth on the Express, but is able to take the place of a Mr Harris who seems to be late.

Poiroit says, “I know my Dickens. Mr Harris will not show up.”

So, who is Mr Harris, what Dickens novel is he in, and why doesn’t he show up?

Sarah Gamp - Wikipedia
From Martin Chuzzlewit
“Mrs. Gamp is dissolute, sloppy and generally drunk. In her long, rambling speeches, she refers constantly to her friend Mrs. Harris as support for her questionable practices. It becomes clear, however, that no such person exists other than as a figment of her imagination.”

Aha. I’ve never read that one. Thanks.