Vacarme translates to "din" or "racket", referring to a loud, confused, and unpleasant noise. It's a more evocative and less common word than simply saying "noise" (bruit).
In the song, Aznavour uses it to describe the "vacarme infernal" (infernal din) made by the train, setting a vivid scene for the unexpected romance that unfolds despite the uncomfortable journey. This word adds a touch of drama and imagery to the setting.
"Départ Express" tells a small love story that plays out entirely on a train. A man boards on a whim, bored with his life, and tells anyone who asks that his destination is "unknown." On the train he meets a woman, and in the cramped, rattling compartment they fall for each other fast: a stolen kiss in one tunnel, another in the next, then a quiet goodbye at a station where they can hide their romance in a small, hidden corner.
One morning, out of nowhere, she tells him to leave, and he does, boarding another train just as "unknown" as the first, a little hurt this time. But she comes back. She takes his hand the way he once took hers, and the same tunnels, the same stolen kisses happen all over again, this time with her leading.
The song's charm is in that mirroring: the same lines, the same train, the same tunnels, replayed with the roles reversed. It ends simply, with the two of them still together since that day, because she turned out gentle enough to keep him.