I first made this journey in August 2014 and it’s a very pleasant and fast one with a quick change in Brussels – London to Cologne can be done in 4 hours 15 minutes, Dusseldorf in 5 hours and Frankfurt in 5 hours 30 minutes.
The London to Brussels leg is described here – there are two operators on the Brussels to Cologne leg, Thalys trains running from Paris to Cologne or German ICE (InterCity Express) trains from Brussels to Frankfurt. When I’ve made this journey it’s been on the 10.58 from London St. Pancras, then the 14.25 from Brussels to Cologne and Frankfurt which is an ICE. Bear in mind the connection is usually only 20 minutes so use the Connections exit from the Eurostar and go straight to the platform for the Cologne train.
The journey from Brussels stops first at Liege Guillemins station, a modern masterpiece designed by Santiago Canatrava and opened in 2009 featuring a huge 160-metre long arch, before stopping at the border city of Aachen and then arriving at Cologne at the Hauptbahnhof (main station) 1 hour 50 minutes after leaving Brussels.
Cologne Hbf is an impressive structure itself (the cover image to this article), but come out the main entrance and you see something far more impressive, the imposing cathedral which is directly opposite the station. One of the great benefits of train travel over planes is that your first experience of a city can be something as impressive as this rather than an out of town airport and a shuttle bus.
If you’re travelling onto Dusseldorf a through ticket from London (which is often the same price as to Cologne) entitles you to use any train except ICE’s between Cologne and Dusseldorf. It’s certainly worth waiting an hour and having a look around the cathedral first before catching your train. Generally speaking the regular Inter Cities and Regional Express are the fastest trains between the two cities, the suburban trains take a lot longer.
There are also some direct Thalys trains from Brussels to Dusseldorf, but these usually don’t connect well with Eurostars from London.
Nearly all the German ICE trains continue onto Frankfurt from Cologne and arrive there an hour or so later, stopping at Frankfurt Airport on the way. Almost the entire route from Brussels to Frankfurt is on dedicated high-speed lines. You arrive at the huge Frankfurt Hbf, the busiest train station in Germany and located very centrally (although the area around the station is not particularly salubrious)
Through tickets to Cologne and Dusseldorf are usually around £80 booked in advance from Eurostar, but if you book through the German Railways website db.com you may find a cheaper fare, and often a first class fare for not much more than standard. You get extra legroom and complimentary drinks and snacks on the ICE trains in first class.
My trip to Frankfurt in April 2018 was booked through DB and a first class single from London was £140 – including Standard Premier on the Eurostar leg.