There are a variety of trains available, with the fast trains costing more. You will probably need to change somewhere. You can get train times and costs from www.trenitalia.it/homepage_en.html and, for detailed information on booking online, see roninrome.com/%20transportation/booking-on-t…
The Trenitalia site will tell you where you need to change.
Note that on http://www.trenitalia.com/homepage_en.html the fares on Regional trains (which include the only direct train of the day from Milan to Riomaggiore) are NA online more than seven days before travel, but cost €13.90.
If you do not understand the abbreviations for stations where you change trains, select the train and click 'Details by selection'.
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