Excess Express waitress
From the Super Mario Wiki, the Mario encyclopedia
Mario first meets the waitress after boarding the Express Express. During Mario's trip to Poshley Heights in their quest for the Garnet Star, her Shell Earrings are stolen by Doopliss (disguised as Zip Toad). The Shadow Sirens planned to mix the Shell Earrings with Toodles's Gold Ring and some Nitro Honey Syrup to set off an explosion, destroying the train and fulfilling the "sticky, yummy doom" Mario has been warned of. Mario eventually catches the fake Zip Toad, who reluctantly relinquishes the stolen items. If Mario returns the Shell Earrings to the waitress, she will reward him with a Star Piece in gratitude.
The waitress's Shell Earrings were given to her by her ex-boyfriend, apparently an unnamed Squeek Mario can speak to in Podley's Place. The waitress often uses star symbols in her text and speaks in a hyperactive, line-by-line sequence. According to Goombella, she probably is not a very good waitress, but likely attracts many men.
On the third day, the waitress gets absorbed into a hoard of Smorgs that attack the train. Mario fends off the Smorgs in the baggage car and free the waitress, along with the train conductor. After Mario defeats the Smorgs, causing them to blow away in the wind, the waitress thanks Mario for saving everybody. She remains aboard the Excess Express for the rest of the game.