I believe the most general and at the same time the most specific term for street, beaches, trains and ships, and eventually planes is “vendeur ambulant” or synonymously “marchand ambulant“. I have never heard of a term used specially for trains.

“Vendeur” is not specific, yet it is going to be a common term if not the most common to refer to such a peddler of eatable goods in trains and other places, and as well for other types of goods (papers, gadgets, hats…). “Marchand” can also be used; if need be, those terms can be made more specific as to the nature of the goods : marchand de cacahouètes, vendeur de glaces, vendeur de chapeaux de paille, etc.

Example of use of “vendeur” in a dialog

— La glace que tu m’a donnée est une glace à la vanille, tu sais que je ne les aime pas.
— Tiens, voilà un euro, le vendeur n’est pas loin, vas acheter celle que tu veux.(Here you are not likely to use the full term (vendeur ambulant), the abbreviation is usual.)

Other examples

• Le marchand de glace ambulant sur la plage ne vient plus dans les après-midi, il faut aller chercher les glaces au restaurant.
• Un vendeur de bonbons passait dans l’allée centrale du wagon avec son plateau de sucreries accroché à son cou par une sangle, présentant celui-ci silencieusement et brièvement aux passagers.

“Colporteur” is not applicable as although a “colporteur” is a “vendeur ambulant”, he does his business by knocking on doors; I think in English one calls those peddlers“. Moreover it is a term that describes an occupation that is not any more that of enough people for its name to have much currency.

(TLFi) Marchand(e) ambulant(e) qui vend des marchandises à domicile
(internaute) Commerce Ancien —
Marchand ambulant qui transporte des marchandises et qui les vend en faisant du porte à porte.