I think that the answer is to be found early in Chapter 3. In my edition, Chapter 3 starts on p. 29 and the quote you need is on p. 30.
In this Chapter, Poirot is hanging out with M. Bouc, the director of the company. They are in the dining car looking at al the people. Bouc points out to Poirot that the scene in the car "lends itself to romance." He then goes on to point out how many different kinds of people there are in the car. There are people of lots of different ages, nationalities, and even social classes.
When he says this is romantic he doesn't mean it in the sense of love and moonlight and flowers. Instead, he is using the definition that just refers to a situation that is mysterious and emotionally touching. He is saying that it is a setting where anything seems possible.