Train Travel in Europe

A few thoughts:
Can you tell us why you're adding Frankfurt into the mix?

Marseille to Paris is better done by train - it's only a little over 3 hours so no need to fly even if you're seeing the same territory as you travel - the train is just simpler and takes you right into the middle of Paris, not the airport.

buy your London to Marseille ticket ASAP - now, today!

http://www.eurostar.com

for your journeys no need for any pass

Look up times and prices on the website of the company running the trains. For Eurostar this is www.eurostar.com
For French internal trains, and some surrounding countries I can also recommend: https://www.captaintrain.com/en

The route a TGV (French High Speed train) takes from Marseille to Paris is the high speed line same as the Eurostar takes from London to Marseille. The Eurostar loops round Paris on a high speed bypass line.
But, allowing for the various delays in flying (getting to and from the airport, check-in times etc.), The train from Marseille to Paris is probably faster than flying.

Another option is London to Paris, then Paris to Marseille, and then fly Marseille to Frankfurt. If you are visiting the Rhne valley area, you could also fly to Cologne (Köln) or Düsseldirf.

Eurostar does not accept rail passes. It does operate that one, direct London-Marseille trip on Sept. 12. But London-Marseille is the longer, more expensive ride, so why not fly that one and just train Marseille-Paris? Or train London-Paris, visit there, then Paris-Marseille, then fly to Germany? The Marseille-Paris trip is on the same track that you would travel on southbound. If you can firm up the plan and buy the longest point to point tickets soon, they will be much cheaper than waiting.

I also would question why Frankfurt or guess that you have other destinations and train rides in mind that you haven't counted up for the price comparison. If you just want scenic train rides, consider Switzerland. A 4-day pass for France and Germany costs $290 if you're 26 or older and traveling alone, otherwise can be cheaper, plus seat reservation fees for the French TGVs, which we also recommend booking ahead. If you have more travel in Germany, then a pass can be used more flexibly there.

British Airways has nonstop flights from Gatwick to Nice in 2 hours, 78 GBP for the afternoon flight. That would be the flight to take. Then TGV to Paris and TGV or ICE to Frankfurt.

I was going to add in Germany as my husband has been wanting to see Germany in forever! I realize it can be done in another trip another year but wanted to include his desires. I have been tossing around the idea of going to Paris first (as long as he get to take the channel tunnel it doesn't matter to him) and since was exploring the Germany option and trains leave Paris it didn't seem realistic to backtrack to Paris for that. I wanted to go to the south of France and since we are that far he'd like to go to Monte Carlo. I know the earlier I purchase the train tickets the better price but would like the itinerary that makes the most sense before I get "stuck" with it! Thank you so much for your help!

Since we haven't taken the train in Europe at all I share your thoughts. I'm learning, too! You could take the Eurostar from London to Paris then down to Marseille. Fly from Marseille to Frankfurt (if flying on the 19th of Sept that flight looks like it would be $115 each), then fly home from Frankfurt. Enjoy your trip!

That's fine, but rather than go all the way back to Marseille to fly to Frankfurt, I'd fly from Nice to Frankfurt (if you do go to Monaco).

You plan to "...take the train to Frankfurt staying a few days before flying back to the US." Flying out of FRA is reasonable. But if you want to enjoy some of the best of Germany, I would encourage you not to spend that much time in Frankfurt, if any. Not that it's a bad sort of place - just that there are more interesting places with more to offer, IMO. Paris-Frankfurt takes 4 hours. Paris-Trier (on the Mosel river) is 3.5 hours and you'll be in Germany's oldest city (Ancient Roman outpost) and at the gateway to one of Germany's most productive and scenic wine regions, the Mosel River Valley, which is dotted with old-world villages and castles between Trier and Koblenz.

http://www.thereareplaces.com/newguidebook/pdest/germany/moselle/moseltal.htm

From Trier you can take the train to Cochem and to Moselkern for the walk to Burg Eltz (see above link for further links) and then to the Rhine River at Koblenz. South of Koblenz lie 40 miles of river valley with 40 more castles and several more worthwhile destinations including Braubach (Marksburg Castle,) Boppard, St. Goar (Rheinfels Castle,) Oberwesel and Bacharach. Bacharach is actually a good place for a final night in Germany - it's only about an hour from FRA by train.

Rick has information on these places in his book and website - and you can find tons more info with a search on these forum pages.