The Rheinland-Pfalz-Ticket (http://www.vrminfo.de/en/...) lets groups of up to five people travel freely for a day 9 AM onwards on weekdays, all days weekends, up to 3 AM of the following day by train, bus and local transport within the state of Rhineland-Palatinate and Wiesbaden for €21 for a single person, €25 for two person and €37 for five person. In addition to the core stretch along the Rhine Remagen-Oberwesel inclusive and the train line along the Mosel, the ticket also covers a few handy train stretches outside state boundaries, notably Rolandseck-Bonn and Koblenz-Rüdesheim-Wiesbaden-Mainz.
By ship
The Köln-Düsseldorfer Rheinschiffahrt (http://www.k-d.de/english...), better known as KD, runs cruises and scheduled services up and down the river between Cologne and Mainz. The summer season May-Sept sees up to 8 services daily on the busiest parts of the river, but services are cut considerably in the shoulder seasons of April and October and slow down to a trickle in the winter. Traveling end to end takes over 11 hours €49 one way, so most travellers opt for much smaller segments: St. Goar to Bingen, for example, passes by the famous rock of Loreley, takes about 90 minutes and costs €15.30.
While KD has the most extensive network and schedules, there is quite a bit of competition. For example, Bingen-Rudesheimer (http://www.bingen-ruedesh...) operates scheduled services on the south half between Rüdesheim and St. Goar.
By train
There are not one but two train lines running along this section of the Rhine. The scenic Linke Rheinstrecke "Left Rhine Line" runs along the left west bank of the river from Cologne to Mainz, while the Rechte Rheinstrecke "Right Rhine Line" runs along the right east bank of the river from Cologne to Wiesbaden. The Linke side, generally considered the more scenic of the two, is more heavily trafficked and has InterCity services, while the Rechte side is mostly dedicated to cargo and is limited to regional passenger trains running less than once per hour. Interchanging between the two is possible at Koblenz; the city is on the left, but some trains running on the Rechte start or terminate in there.
Beware that, if you're buying individual tickets, train zones get confusing and pricy fast. While the "core" of the Rhine Valley is in Rhineland-Pfalz's VRM tariff zone, the Rheingau stretch east of Lorch is also in Frankfurt's RMV zone, while going north of Remagen passes into Cologne's VRS area.