Best car rental spot in Koblenz from main HBF station?

Koblenz to Köln isn't very far - can you get the car at the airport or Köln Hbf? Many outlets are closed on weekends or have short Saturday hours - that's less likely at the airport or big Hbf. If you start down the Bonn side of the river you can even visit the Drachenfels and Siebengebirge on the way up or down.

We have a 4 year old so I was thinking getting a taxi with a car seat would be difficult from the train station to a car rental spot.

Normally taxis have children seats available.

Is Uber an option?

No. And the children seats are mandatory by law for all car transports of children.

Do any car rental agencies pick up and drop off at that train station?

Give them a call or ask via contact form, e. g. Sixt, Europcar or Avis.

Conclusion: rent at Hertz or just take a taxi to one of the firms.

I picked up a car for our party of 5 at a car dealership just a short walk from the Koblenz Hbf station... roughly 25 years ago... but as I recall, I had to make a special arrangement with the company to avoid making the trek to their location on Andernacherstrasse for that.

I looked at your previous posts to learn more about your plans and your concerns. You wrote...

A train would totally be possible, but with the weather in question
that time of year (mostly concerned with rain), I’d like the car for
luggage and not some of the longer walks from stations.

You also said you were leaning toward the Bellevue in Boppard. Are you booked there? If so, there are direct trains from Cologne that will drop you at Boppard Hbf, and since Boppard is a small town, you'll have a very short walk to the Bellevue (and most other Boppard hotels.) The Bellevue is the red pin on this map:

https://1033609670.rsc.cdn77.org/maps/le-chopin-im-bellevue-rheinhotel-boppard-map.jpg

As for the rain, there is no rainy season... there is almost zero difference from month to month in terms of total precipitation and rainy days. The area gets about 2.2 inches of precip during the entire month of March, on average, and 70% of the days in every month see no rain.

Your plan was fairly basic - to "Drive Rhine route, stopping at various towns, castles etc,"

For visits like yours, I have no hesitation about recommending the train. The train schedule in Boppard and most Rhine towns is more than ample. Just this morning, Boppard sent off the usual number of southbound trains to places like St Goar (Rheinfels Castle,) Oberwesel, and Bacharach between 8:15 and 12:15 - 9 of them. The northbound trains have the same frequency. And every single riverfront town has train service. You can see all the train stops on the railway map on this side of the river between Cologne and Mainz:

https://www.duesseldorf-blog.de/wp-content/uploads/2008/12/karte.gif

In Koblenz, just north of Boppard, the station serves BOTH sides of the river. This makes visiting Rüdesheim, or Marksburg Castle in Braubach, on the opposite riverbank from Boppard, a fairly easy matter. Here's a map of that railway showing Koblenz > Frankfurt:

https://rheingaulinie.de/files/rheingaulinie/files/Fahrplan/fahrplan.png

Ticketing is easy. There are day passes which provide for hop-on, hop-off travel and cover all destinations. But if you are staying in BOPPARD, your hotel will give you a train pass - for free - that's good for your entire stay - the VRM guest ticket - which makes the rental car idea even less competitive.

Burg Rheinfels Castle in St. Goar: Bus 681 will take you up to the castle from the town center. Buses are covered by the VRM guest ticket and other day passes as well.

You may require a short taxi ride for touring Marksburg Castle in Braubach:
http://funktaxi-braubach.de/kontaktzuuns/index.html

Nigel, I thought about that, but I figured less driving near the bigger cities would be better. I thought a car from Koblenz would be easiest, but maybe not. I do already have our train ticket, but we could always eat the extra cost I suppose If we end up just getting a car in Cologne.

MarkK, I've never taken our little guy on a taxi so wasn't sure. That's good to know they should have boosters, but I suppose I can double check when we call, or ask at the station. I'll try contacting some agency's to ask about pick up and drop off options.

Russ, our plans have changed quite a but from that earlier post.
So our new plan is to train from AMS to Koblenz, Rent a car for 2 days while staying near Sankt Goar and exploring Rhine and a little of the Mosel.
I know a train is possible for the Rhine, but I wanted the flexibility of a car and I would like to see Burg Eltz which will be easier with a car also. I'm less stressed about the rain as we've gotten closer to the trip, I'm excited for the unknown.

I thought a car from Koblenz would be easiest, but maybe not. I do already have our train ticket, but we could always eat the extra cost I suppose If we end up just getting a car in Cologne.

You can probably still pick up in Koblenz without chucking your pre-purchased train ticket.

The DB site permits not just stations as destinations but addresses as well - including addresses for car rental locations. The train ticket that you already have in hand is all you need if in fact you are able to arrange your pick up at the following address:

Koblenz, Avis, Beatusstraße 20 (Mietwagen) (I believe this is the same location where I picked up a car once.)

The DB details show Koblenz Hbf as the intermediary stop to this address - and then show a walk of 306 meters to this location. This would be a snap. Two of you wait at McDonalds (or elsewhere at the station) while the other walks to the rental location and then picks those two up.

If you cannot make this arrangement, then try inputting whatever Koblenz address you have for car rental location in question. Your train ticket will still get all of you to Koblenz Hbf. Then one of you will taxi or take a bus (pay additional fare to jdriver) to the rental location. I notice that that the DB site ALSO has the following addresses on Andernacher Str. for Europcar and Avis as pull-down choices:

Koblenz, Europcar, Andernacher Str. 199 (Mietwagen)
Koblenz, Avis, Andernacher Str. 203 (Mietwagen)

The DB details show which bus one of you would would need to take - and the distance for any additional walking needed from the final bus stop. Or just taxi to that address.

After more research, there will be no car rental agency’s open when we arrive on a Saturday at 2:30 in Koblenz and haven’t found one yet that has an after close pick up option. So now I feel like we are stuck eating the cost of the train ticket and getting a rental in Cologne. Thanks everyone for your advice.