Best Rail Ticket For Rhine Travel Day

The RMV ticket works for getting to Rüdesheim, plus it will get you a 20% discount on the ship. If you want to get that ship in Rüdesheim, at 09:00, you will need to get the train from Frankfurt that leaves at 06:53. The train arrival and the ship departure do not match up. Alternative is to ride the train one more stop to Assmanshausen and get on the ship there. You could catch the 07:53 train then.
I would just get the regional train ticket from St. Goar to Koblenz.

For 4 passengers there is the Quer durchs Land ticket, which costs 73 EUR for 4 people, but it must be used after 9 am on week days, but any time weekends and holidays. You do the calculations to see if this if cost effective. Only local and regional trains.
https://int.bahn.de/en/offers/regional/day-ticket-germany

Can I buy the RMV ticket at any station. After looking more carefully, we will need to ride the S-Bohn from our hotel to the HBf. Could I buy the RMV at the S-Bohn station or only at the HBf?

Or could I buy the RMV ticket the night before when we arrive at Fra HBf? And I couldn't figure out the pricing either for the RMV group ticket. Do you know what the rough cost would be for me so I can budget? 2 adults, 2 kids 16 & 13. Konstablerwache station to Rudesheim/Assmanshausen

Also, am I doing the math right?

If I buy a one way ticket from St Goar to Koblenz I am 10.80 x3 and 6.50 x1 (kids are 13 and 16). So I would be 38.90.

And the Rheinland-Pfalz-Ticket is 42 but would then allow me to travel to other cities on the river, correct?

Yes, you can buy your ticket to Rüdesheim at the Konstablerwache. Since it will be a group ticket, you can buy it the day before if you like. Remember to write your names on the back of the ticket. Cost is 40.50€

The Rheinland-Pfalz ticket will let you ride all day long on that side of the river.

If you are going on a weekend, the Germany wide ticket is your best deal, just not on weekdays since you cannot use it before 09:00.

Then from St Goar we need to get to Koblenz later in the day

if there is a good group train ticket, we my use the train to explore a little more to the south... the Rheinland-Pfalz-Ticket is 42 but would then allow me to travel to other cities on the river, correct?..

I don't see any mention of returning to Frankfurt for the night. Is that your plan? Or do you have a train to catch in Koblenz? Are you staying there for the night? Will you be bringing bags with you on the trains and cruise boat? And do you intend to sightsee in St. Goar (Rheinfels Castle?) Is there a plan in place for managing your bags, if so, or if you are planning to sightsee in nearby towns (Bacharach? Oberwesel? Boppard?)

If you are returning that night to Frankfurt... what are your plans in Koblenz, and what time do you need to be back at Konstablerwache?

Is it actually critical that the 4 of you board only in Rüdesheim and take only the 9 am boat (and thus all be boarding at Konstablerwache at 6:53 am?)

Sounds to me like your plans for visiting and exploring along the Rhine might be too vague at present to plan your ticket purchases. Besides the 1.5 hr cruise, what do you hope to accommplish this day? Also that your 6:53 am deprture from Frankfurt might be unnecessarily early and burdensome.

If they want the 09:00 ship in Rüdesheim, they need to be at the main train station to catch that train at 06:53, not from the Konstablerwache. The train going to Rüdesheim leaves every hour from the Hauptbahnhof at 07:53, 08:53, 09:53, etc.

If you take the train back to Frankfurt from Koblenz on the right side of the Rhein, your Rheinland Pfalz ticket for the day is valid until you get to Hessen (Rüdeshiem/Assmanshausen), and then your group ticket from that morning is valid again.

Right, so leaving from K'wache will mean even earlier rising and earlier train boarding, something most 13-year-olds IME might divorce their parents over... my point is that MAYBE none of this early-morning, breakfast-threatening fury is necessary at all and that some entirely different schedule on this day would work better. Just depends on the family's true parameters on this day and on their specific interests beyond the cruise.

Here are the further details as requested.
We will be doing our Rhine day on Thursday June 13. We will not be returning to Frankfurt, we will be renting a car in Koblenz to head to the Nurburgring. The rental car place closes at 6 so at the latest, I will need to be on the 5 o'clock train from St Goar but may take the 4:00 train to be sure I am there on time as there is a bit of a walk from the train station to the Sixt location. (google says it is a 22 minute walk from Lutzel station to the Sixt location which would be cutting it VERY close considering the train arrives at like 5:32)

We will have bags which is not ideal but plan to leave them at the St. Goar TO as others have suggested.

As far as the timing, we like the idea of getting on the boat at Assmannhausen which allows us an extra hour in the morning. Getting on that first boat is not essential, but if we can, it allows us more time flexibility for the rest of the day...which would include visiting another town besides St Goar or perhaps taking the cable car in Koblenz. If we do explore another town, we will need to also return to St Goar to collect our bags and wait for the next train.

But if we get the first KD vessel, we will arrive in St Goar with plenty of time to explore the castle, meander, and hopefully have a couple of hours left over to check out another town (open to suggestions too...My wife and I have only been to St Goar and wanted to show our kids the castle, so something new would be awesome)

Unfortunately, the St. Goar TI office is not open to take your bags until 14:00 on Thursday:
https://www.romantischer-rhein.de/a-tourist-information-st-goar

To be on the first boat I suggest catching the S-Bahn + RB train sequence from Konstablerwache as follows...

7:11 Lv K'wache
7:56 Ar Mainz Hbf
8:03 Lv Mainz Hbf
8:31 Ar Bingen Rhein Stadt; walk 5 minutes to KD dock and buy tickets.
9:15 Board K-D boat for St. Goarshausen (Ar 10:45.) Walk north to train station (10 min.)
11:32 - Train to Koblenz Hbf (Ar 12:06)
12:19 - Train to Lützel (Ar 12:23) to get car while family keeps bags and has lunch at Koblenz station.

Now with the car you can drive to Rheinfels and the other places you might wish to visit and be on your own schedule.

I don't really like the idea that you must get up so early and transporting yourselves from Frankfurt. The better choice would be to wake up in Bingen or Rüdesheim on the morning of the 13th, have a decent breakfast, then get on that 9:15 (or 9:00) boat.

The nh Bingen hotel has a fantastic setting with great views of the river gorge and is right on the promenade where the K-D boats leave. Very nice breakfast too.

RMV group day ticket for K'wache > Bingen = about €40 for all of you.
VRM mini-group ticket for St. Goarshausen > Koblenz = €30 for all of you.

These tickets can be bought on their respective apps.

Show the RMV ticket at the K-D dock to get a 20% cruise discount.

Another reasonable choice...

Pick up the car earlier in Frankfurt and drive it from K'wache to Bingen, where you park (bags stay in the trunk) and begin the cruise to St. Goar. After the cruise, tour Rheinfels. Then ride the train BACK to Bingen to fetch the car.

Excellent suggestion, thank you @russ!

I will have yo make sure I can pick up the car earlier than I booked it but that shouldn't be an issue.

And we were originally going to stay in Rudesheim but we were able to secure free hotel stay at the posh Westin in Frankfurt where we favored better beds and a good night's sleep vs risking a lesser bedding situation in a quaint hotel.

Update.

We ended up following Russ' plan which worked out in many ways but wasn't without it's hiccups.

We boarded the S8 for Mainz on time after easily getting my RMV Ticket, but of course the train was late so we missed the 8:03 to Bingen. There is, however, also an 8:33 which we discovered while at the station trying to figure out our options. This train would have still gotten us to Bingen with 10 or so minutes to catch the boat...but it was also late.

As the train pulls into Bingen, we see the boat pulling up to the dock...so we basically sprinted the 400m to get to the ticket window and she held the boat for us so we still made the boat we wanted.

Picking up the car early was a great option and worked out for the better because the car rental office contacted me to say that they needed to close early that day too.

After reaching St Goar, we were deciding how to best approach who all should go to fetch the car vs just letting them stay in St Goar with the bags.

Discovered the that tourist office WAS open and didn't have the odd hours for that Thursday like their site had mentioned (hours on google were correct). But I think they must have JUST changed the hours for the summer or something. They were very nice and helpful and happy to store our bags. They scoffed at the suggestion that we would need to pay for the service. Very grateful.

So we left the bags there, the family went to lunch in St Goar and I went to get the car. Now that it's only me, saved a bunch on further rail tickets and now that we had the car, we saved our legs/a few euro getting up the hill to the Berg now that we had a car.

Discovered while at the castle that Rick Steves himself had been there giving tours just 1 day prior. Would have fan-girled quite a bit had we been there at the same time.

We weren't able to take the time to visit any other towns along the way but took some time to visit Koblenz for dinner before we headed to Nurburg for the night.

Thanks for all the help everyone!

Thanks for reporting back. Always interesting to hear how things went. Where did your crew find lunch?

I think your experience is probably instructive for others. Some of Rick's materials make it seem like you can swoop in from some distant place, take a cruise, see Rheinfels, then swoop out to some other distant place, with everything going as you planned it. But the truth is that a visit which mixes cruise boats with cars with trains over just a few hours is inherently iffy, and that today's weak on-time performance on the part of Deutsche Bahn makes things even iffier sometimes. I don't think DB should get deserves kudos for helping you with your sprinting skills.

I always try to encourage visitors to spend a night or two in the area... St Goar is a really good option for a one-nighter. This tactic will leave you with a fuller impression of the area but will also provides a time cushion to iron out the transport/connection wrinkles if you have to head somewhere else soon. 3-5 days is of course preferrable considering the large number of things to see and do between Bingen and Koblenz. Some of us on this forum have done 3-5 days there on 3-5 different occasions!

You are correct...the way Rick speaks of the KD boats is like a hop on hop off bus tour...and they are not scheduled that way at all.

We learned that the first time we went and should have remembered that this time. But since we were traveling with the kids, we didn't want to spend as much time as we would have if it has just been her and I for a second trip.

Next time, we will def. plan to spend a night in at least one town. I love the feel of the sleepy old towns like that.

It was great to have the car on this trip as well though as we decided to drive up above St Goar to see what the modern town looks like. Cool to see that what you see from the river is not the current model of what sustains these towns.

But you are right again. DB punctuality is not what it once was. I don't think I was on one single on time train the whole time in Germany. The day we dropped the car, it was late in Munich and I went to catch the U Bahn back and the whole U Bahn system went off line...so I had to search for a cab late at night. Glad it was just me since I dropped them at the hotel, but I learned from the cab driver that the status of DB and the rail systems are not up to standard.

As far as the meal in St Goar, they went to Alla Fontana and were very pleased with the food. I got the luke warm left overs and they were still very tasty. It was, surprisingly, cash only. But a wonderful lunch and let them rest a bit from the chaos of the morning.

Some additional feedback for any future readers who may come across this.

The weather was perfect on our day on the Rhine....BUT....since we caught the boat in Bingen, there were literally NO seats left on the top of the boat which is obviously more ideal to try and see as many of the sites as possible. And keeping the glass clean on the KD boats is not a priority so I found myself going back and forth to get good pictures/video and then sitting with my family.

So catching the boat in Rudesheim vs Bingen (and being early to be at the front of the line) would have offered a better cruise experience (though my youngest would have freaked out at all the little bugs that are up there...even though they don't at all bother any one).