You could say “I eat” but you’d rather say “I’m eating” if you’re asked what are you doing right now ?
In French, we would say “Je mange” but we don’t have the straight equivalent for “I’m eating” which implies an action being done at that moment.
The French would say “Je suis en train de manger,”
but what’s the matter with a train, here ???
This train is not the one with the carriages and the engine 😉
It comes from the 15th century where “train” had the meaning of action, or pace. Being “en train“ actually means being “in action.” And the French stil use “train” with this meaning but in this very specific case.
“Etre en train de“ belongs in the everyday language meaning being right in the middle of an action.
And if you answer someone that you are “en train de manger”… it very often implies that you won’t be available until you’re finished with you meal 😉
When you are on board a train, you won’t stop down before the end of the trip… anyway ;-)… even if it’s not that “train.”
To go one step further, “entrain” is a word meaning enthusiasm, animation… and it’s very much a today’s word !