Assoviar means "to whistle". While whistling is a common action, the word itself isn't as frequently featured in song lyrics as more direct verbs of communication or emotion.
In the context of "Seguindo No Trem Azul", it evokes a sense of carefree joy and imagination. The lyrics state, "Toda vez que for assoviar, a cor do trem é da cor que alguém fizer e você sonhar" (Every time you whistle, the color of the train is the color someone makes and you dream). This ties the simple act of whistling to the power of imagination and personal creation, making it a charming and unique word choice for the song's theme of love and dreams.
Roupa Nova build "Seguindo No Trem Azul" ("Following On The Blue Train") around a confession the narrator can barely finish: "Que em você, encontrei inspiração" ("That in you, I found inspiration"). He admits up front he doesn't have the right words yet — "Mas não sabe muito bem, Como vai dizer" ("But doesn't quite know how to say it") — so the love itself becomes a color that shifts with whoever's imagining it: "É da cor que alguém Fizer e você sonhar" ("It's whatever color someone makes it, and you dream of").
Rather than force the feeling into one finished line, he offers to share the pen instead: "Eu empresto um verso meu, Pra você dizer" ("I'll lend you a verse of mine, for you to say"). What he's really proposing is company for as long as the ride lasts — "Se viajar contigo, Até nascer o sol" ("If I travel with you, until the sun comes up").