The Sandwich TGV is defined by where it is eaten rather than by what is in it: on a high-speed train, at a tray table, somewhere between two cities, bought from the bar car or carried on board in a paper bag from the station. The contents are the familiar national set, jambon-beurre, poulet-crudités, thon-mayonnaise, a mixte of ham and cheese, but the defining facts are the cellophane wrapper, the chill it was held at, and the fact that it is being eaten in motion with no alternative within reach. It is named for the train, and the train is the whole story.
The craft here is a logistics problem, and reading it that way is the point. A train sandwich is assembled before the journey, held cold in a bar-car case or a station fridge, and bought by someone who is captive for the next few hours and has no other option. Everything follows from that. The bread suffers first: a baguette that was crisp when it was filled has spent the trip pressed against a moist filling behind glass, and by the time it is unwrapped the crust has gone soft and the crumb has gone damp. The better versions fight this with a sturdier bread, a drier filling, and a layer of butter or a leaf as a moisture barrier between bread and contents. The variable quality is not a flaw in the category, it is the category. The good ones were built that morning with real ingredients and a structure designed to survive the case; the poor ones have been waiting since the day before.
Variations are mostly a question of which familiar filling got the wrapper, and the same physics applies to all of them. The ham-and-butter version holds up best because cured meat and fat resist the damp. The chicken-and-raw-vegetable version disappoints most often, because the crudités weep and the bread drinks the water. The cheese mixte sits between the two. What to look for is not a different recipe but a fresher assembly: a date you can read, a crust that still resists a little, a filling that has not soaked through. It is a close sibling of the Sandwich SNCF, the platform and station-hall version of the same idea, and it belongs with the place-and-context-keyed sandwiches the catalog groups under Regional Specialty Sandwiches. Its specific contribution is the onboard sandwich as a category set by the case it traveled in, where the cook's real job is to build something that survives the trip.