Luggage that doesn't exceed 50 cm in any dimension is free. That's basically the size of a typical carry on. Over 50 cm, you have to buy a ticket for the luggage. Luggage over 90 cm is prohibited. Now this is on the municipal city subway. There are other trains where these limits don't apply, but they are more expensive. In any case, for a short hop from Garibaldi to Centrale I doubt you'll run into any luggage problems unless you are bringing a whole lot of luggage. Since these are city trains they don't want you cluttering up aisles, having luggage get in peoples' way. So depends on what you're bringing.