I highly recommend to choose a tour that includes a visit to wine cellars. Regardless whether or not you drink port or wine, the tour itself is quite fascinating inside the cellar caves.
Most train tours are not a "hop-on hop-off" version, so if this is what you are looking for, please make sure to clarify before you purchase the tour.
Almost all train tours allow you to upgrade your tour to include a lunch. Lunch is usually a buffet style restaurant featuring typical Portuguese cuisine.
Most train tours depart every 30 minutes from Sé Catedral (Porto Cathedral), which is located in the old city center.
Onboard train audio guides are good quality and available in many languages.