Bruxelles / Brussels 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.

B-Europe is the website provided by Belgium's national railway operator for the sale of international rail journeys from and to Belgium.
Therefore tickets can be booked for the following international train services to/from Belgium

• Eurostar
• IC trains to France, Luxembourg and The Netherlands
• ICE trains to Germany
• Thalys trains to Germany, The Netherlands and Paris
• TGV trains to destination in France other than Paris
  B-Europe offers a 'Best Fares' policy for journey by these trains.

It also sells international end-to-end journeys solely by train, with connections outside of Belgium, if you will be heading to popular destinations in Austria, Germany, Switzerland and Italy.

Users of Eurail and InterRail passes can also book seat reservations on the Eurostar and Thalys services without paying booking fees.

Happy Rail is a Netherlands rail ticket agency which sells tickets for both national and international journeys within a range of countries including Belgium France, Germany, Sweden, Switzerland.

HappyRail doesn't charge booking fees in the conventional sense, but if you're not Dutch you can expect to pay additional transaction fees, which will be added to the total cost.
That's because the most common form of online payment used in The Netherlands is 'iDEAL' and HappyRail doesn't charge a transaction fee for 'iDEAL' payments, but only Dutch residents can sign up to 'iDEAL'.
Other forms of payment include Visa debit cards will incur a fee.