The are 7 different ways across 3 unique modes of transportation to help you travel from Köln to LGG. The fastest way out of Bonn towards Liège is by bus, as it is the quickest to the LGG area. This doesn't necessarily take into consideration traffic or weather - so these may impact some of your decision making. It's common to find tickets for as low as $21. SNCB operates from Köln to LGG, so you book your trip with them! Book as soon as you can, as there may be limited availability.
The trip to LGG is an excellent trip for the day or a nice trip for the weekend from Köln. The cheapest way to get from Köln to LGG is $22 and the quickest way takes just 2 hours 23 minutes with the bus.
While buses have recently gotten bad coverage because of cleanliness of bus stations or winter time accidents, they are can be very convenient for travel. The best part about traveling by bus to LGG is that it can be very cheap. That's because they usually have basic accomodations - WiFi and a restroom. Some of the Köln/LGG buses' coverage has been overstated. The real thing to consider when taking a bus from Köln is to consider the time of travel and the density of the cities that you're traveling to. If you are traveling by bus between Köln North Rhine-Westphalia and LGG during high traffic times, you can expect to have your trip altered significantly due to traffic. Further, if you are traveling during winter, the bus schedule might be thrown off due to inclimate winter weather. Be sure to plan accordingly!
The plus side of taking the train from Köln to LGG? The accomodations, scenery, and lack of LGG traffic can usually be on point. Moreover, you can usually expect your average concessions - such as food and drinks. Most trains have WiFi these days as well. The worst part about trains are the price. While the train to LGG is often cheaper than flying, they are generally more expensive than buses. Train prices are usually set and do not vary as much as bus prices or flights or so the likelihood of finding discounts is pretty rare.