The concept is quite different from how we usually use trains for bike rides in the countryside, and not just because you don’t have to wrap the bike. I think for a lot of people the bike bag will not even make a difference because they live too far from Ryogoku, they will be using a bike bag on the way to and from Ryogoku anyway. So instead of unpacking at the beginning of the ride and repacking before they will get on the train, they will be doing that when they change trains at Ryogoku (I kind of doubt those special bike racks will work with the bike in a rinko bag).
Looking at the PDF, the trains will run on Saturdays and Sundays only, with a different pre-selected fixed course (Uchibo, Sotobo, Sawara or Choshi, same one for that Sat+Sun) every weekend. So you basically pick the date by the destination of the ride, or vice versa.
Each day the train will only return from one particular station at one particular time in the afternoon, but there will be 1-3 different stations on the way out where you can get off in the morning. So it’s one train out, one train back. This is clearly for cyclists who like to stick to a particular schedule, regardless of the weather or other circumstances (including, making a reservation for the train before you know the forecast for the weekend).
We’re used to jumping on the first train after we get back to the station. If the weather doesn’t cooperate or we have technical issues or we run behind schedule, we may bail out from a different station. Not so with Boso Bicycle Base.
I don’t really get why this is a cyclist-only train. It must work out prohibitively expensive to JR East in terms of capital costs if it only gets used for 4 runs (out/back, Sat/Sun) per week. What’s the payback period? That doesn’t scale.
On the plus side, by creating a “cyclist ghetto” event you won’t bother non-cyclists with the smell of your sweaty jersey… Not that I think the people on this ride will get very sweaty, with a whopping 300-450 m of elevation gain on the most challenging routes suggested (50-65 km).
Presumably there is a target group for this kind of train+cycling event, but I don’t think it includes me or most of my cycling friends.
I second @microcord’s preference for the Tokyo Wan Ferry from Kurihama. The ferry ride is scenic and fun. I recommend it as an experience, even if you weren’t going to cycle in Boso after the crossing! Once I did the trip via Kurihama “no rinko”. At other times I did the rinko thing to and from Honda station. Either way, Boso is a great place for cycling.