Marseille to Frankfurt by train

DB is the national railway operator in Germany, so its website can be used for booking journeys by German express trains; the ICE and IC trains and it doesn't charge booking fees.
It also sells tickets for journeys by direct trains on all international routes from Germany regardless of whether DB is operating the train service.

It also sells an extensive range of end-to-end journeys which involve making connections both within Germany and in neighboring countries, but journeys between Germany and Britain cannot be booked on DB.

A key feature of DB website worth keeping mind is that it offers 1st class ticket purchasers complimentary seat reservations on journeys both within and to/from Germany.

Seat reservations for daytime in both 1st and 2nd class can also be booked separately from tickets.

SNCF is the national rail operator in France and not so long ago it recently simplified both the range of tickets and the term and conditions of using them.
Therefore the cheapest tickets also become more flexible too.

Other improvements included a simple integration of travelling with bicycles and rail pass users being able to book reservations for national journeys, without paying booking fees.

Though a legacy of when SNCF sold international tickets under the Rail Europe branding is that OuiSNCF is available in multiple translations with the offerings on the site then tailored to international visitors to France.
However, some of the site's features, including the integration of mobility assistance into the booking path, are only available on the French language version.

Something else to look out for is that simplest way of travelling with tickets is to use the SNCF 'My Travel' phone app, so it's therefore often a good idea to book on the OuiSNCF mobile service.