The Sandwich SNCF is defined by where it is sold rather than by what is in it. This is the train-station sandwich: the wrapped triangle or the half-baguette pulled from a refrigerated case on a platform, in a station hall, or from a trolley moving down the aisle of a carriage. The contents are the familiar national set, jambon-beurre, poulet-crudités, thon-mayonnaise, mixte ham and cheese, but the defining facts are the cellophane, the chill, and the clock. It is named for the point of sale, and the point of sale is the whole story.
The craft here is really a logistics problem, and understanding it is the point. A station sandwich is made hours before it is eaten, held cold, and bought by someone who has no other option and no time to be choosy. Everything follows from that constraint. The bread is the part that suffers first: a baguette that was good at 6 a.m. has spent the morning trapped against a moist filling behind glass, and by the time it is bought the crust has gone leathery and the crumb has gone damp. The better versions fight this with a sturdier bread, a drier filling, a layer of butter or leaf as a moisture barrier between bread and filling. The variable quality is not an accident of the category, it is the category. The good ones are the ones assembled that morning with real ingredients and a build designed to survive the case; the bad ones have been behind the glass since the day before.
Variations are mostly a matter of which familiar filling got the cellophane treatment, and the same physics applies to all of them. The ham version survives best because cured meat and butter resist the damp. The chicken-and-raw-vegetable version is the one most likely to disappoint, because the crudités weep and the bread takes the water. The cheese mixte sits in between. The thing to look for is not a different recipe but a fresher assembly: a date you can read, a crust that still has some resistance, a filling that has not soaked through. It belongs with the place-and-context-keyed sandwiches the catalog groups under Regional Specialty Sandwiches, and its specific contribution is the travel sandwich as a category defined by the case it waited in, where the cook's real job is to build something that can survive being bought late.