I just took the train from Paris to Mainz last month. It was a Thalys train from Paris Nord to Koeln (Cologne). Real nice train, 1st class ticket was 91€, meal and wifi included and I think 1 rebook allowed if plans changed. Booked through the SNCF website, but I had French speaking help. We left Paris at 1001 and were scheduled into CGN at 1315. Missed the connection to FRA (snow) in CGN but took a local train that actually stopped in Mainz on the way to FRA Main HB. Evidently there is non-stop fast service from CGn to FRA main and then you have to take a short hop back to Mainz or, as I did, take a slower train with a number of stops and get off at Mainz before it gets to FRA. Those trains run quite often and they honored my ticket no problem. The train I took was slower but took a spectacular route right along the Rhine and was great. The second leg was 105€. Good luck.