We took bikes on Eurostar last month, including the box option for one bike. A nightmare! I had to remove both front and rear racks and then the purpose-built box was not wide enough to accept the wheels in with the frame. They supplied a second box for the wheels at no extra charge - it seems they are familiar with this problem. Then there was the right pain of putting it back together on a railway platform. There were 4 of us so plenty of people to guard luggage, but I would hate to do it on my own.
For the return trip, we changed our last night’s accommodation from Ghent to Brussels and took the bikes to the station to be transported whole the night before we travelled. They left on the 8pm train, we were on the 11.56 the following day.
The people operating the system were very amenable and supplied kit for our use, but it is a lousy system.