originally, as a noun, “le train” is “the going motion” of a horse, a human, it’s still used in “arrière-train” to designate the back legs of a quadrupède. “Aller de bon train” = to walk briskly.

In automotive, the “train avant” and “train arrière” are the front and rear axles.

There are other expressions like “le train-train quotidien”, meaning the daily grind.