We are currently in Dusseldorf and want to head to Cologne tomorrow for a day trip. I see the train times and cost for 3 adults and a 7 year old but I don't really want to be stuck to a specific return train if possible. Can we buy them at the station and return whenever? Will it be about the same cost as what I see on the DB site?
unless you buy from a window at which time a small added fee will apply - use the machine.
Do you want to go before or after 9am?
Or use the app.
Yes, you can buy tickets and take any number of regional trains. You can buy a VRS price level 5, group day ticket good for up to 5 people traveling together. Cost is 37.20 EUR. Good for all public transport for 24 hours. So that is good for trip to Cologne, local transport while you are in Cologne, and the return trip to Duesseldorf. Be sure to only ride regional trains, no ICE, IC, or EC trains.
This ticket is not restricted to traveling after 9 am.
Those multi-person regional tickets in Germany are such a great deal it makes me sad to be a solo traveler!
Unless your travel includes a "long distance train" (ICE/IC/EC), you will not find a fare listed on the DB site. If so, you will see a rather expensive full fare and maybe a less expensive "Savings Fare", which will have a "specific train" restriction. But there are other options. Düsseldorf and Köln are both in the Verkehrsverbund Rhein-Ruhr, a transit district. The fares for regional trains, S-Bahn, and local transit are controlled by the district, not shown on Bahn.de, and probably less. Look for fares on www.vrr.de.