Frankfurt or Cologne for rail transfer

At the risk someone will disagree ... I think Frankfurt would be easier. The platforms all run to a main hall which is on the same level as the platforms. It is a bigger station though, and so if you are in a hurry between connections, it might take you longer to get to the next train. Cologne on the other hand has a tunnel from the main concourse that runs under the tracks, and you have to go up a flight of stairs to access the platforms. I can't remember the lifts but there would be some for the platforms as Cologne is a major station.

Can I suggest you download the DB (Deutsche Bahn) app? It has a lot of current train information including timetables, station info, etc.

May I just ask - which train were you thinking about taking? Most trains go through Aachen and Düsseldorf or up through the Netherlands, and Frankfurt seems out of the way for this journey. Cologne seems better fit, at least for able-bodied people.

Lavandula

At Cologne Hbf, looks like you will be going from Track 2 to Track 4, which means a platform change, both of which have elevators. At Frankfurt Hbf, it is a terminal type station, so you walk to the platform head, then walk to the new platform and out to your carriage, so more walking but not up/down levels.

Total travel time is about the same, with Frankfurt about 10 minutes faster. Both appear to have 30+ minute change times, so that will be comfortable.

Cologne has only 12 platforms, while Frankfurt has 33. But just as important is the amount of time you will have to make the change. So a 30 min change in Frankfurt might still be easier than a 10 minute change in Cologne. Note that the DB website will show, under 'details', which platforms the trains arrive at and depart from, so you can judge how far apart they will be.

the first thing to note is that you should use your actual (or likely) travel date in bahn.com when checking schedules. Those who have just done a quick check of this journey without changing the date will have seen amended schedules caused by something happening between Cologne and Düsseldorf over the next few days. Normally you would not use the Aachen - Düsseldorf line (and just to mess things up that now has it's own problem!!).

Frankfurt is off course but because of the way the high speed network is built it can still give a faster journey - and because Sparpreis fares are based on what they think people will pay it doesn't automatically mean more expensive. Frankfurt is a terminal station with 24 platfroms (the rest are underground and not your problem). But being flat doesn't mean easy because you may have to walk all the way to the front of the train, then a liitte sideways and then back along the second train to wherever your seats are! https://www.bahnhof.de/en/frankfurt-main-hbf

Cologne (Köln Hbf) is a stop on the Frankfurt trains. https://www.bahnhof.de/en/koeln-hbf Here you have to use the lower passageways to change platforms and this are positioned around one third/two third the length of the platforms so walking distance might be less. www.bahnhof.de/enkoeln but make sure you don't have a connection from Köln Messe because that also the other side of the river.

Amsterdam is not a terrible idea if you can get the connections to match, it's probably the easiest to change at but will be a longer journey.

There is the problem of getting a fare, the Bruxelles-Köln-Frankfurt and Frankufrt-Berlin trains are DB ICE high-speed, so DB can give instant Sparpreis fare quotes when matched wih most onward trains. But the alternate hours train from Bruxelles to Köln are run by Eurostar which messes up through ticketing. And some Köln-Berlin trains are run by Flix, and should be avoided even if they can offer a through ticket!!! Some Brussels-Amsterdam train are also Eurostar, with a DB connection to Berlin, but by tweaking the connection parameters you can swap Eurostar for an alternative which DB can issue tickets for..