I have 2 touring bikes. One is a Bike Friday New World Tourist and the other is a Waterford Adventure Cycle with S&S couplers. Both can be disassembled (Waterford) or folded (BikeFriday) and put into a hard-sided case for travel. I have uncoupled the Waterford and taken in a car's trunk numerous times. I have folded the Bike Friday, put it in a bag, and taken it on trains.
The Waterford is a full-sized bike that comes apart with a good deal of effort (90+ minutes to disassemble /reassemble). It rides like a standard bicycle and it is my preferred touring bike. However, traveling with it is a drag. The bike case is square and only has 2 small wheels on one edge. This provides only the bare minimum of mobility assistance for a 50 pound case. Since the Waterford is a normal sized bike, all gear carrying methods work without problem. These days, the Waterford stays at home in the US.
I have family in the UK and often travel there to visit. I use these visits to tour in both the UK and Europe. In the past, I brought the Waterford. But, tiring of the travel hassle, I bought a folding bike to leave in the UK. When I tour in Europe, I fly to the UK, pick up the Bike Friday, and fly on to my European destination. The Bike Friday comes in a large, but sturdy suitcase with the usual modern wheels and handles. It is easy to roll through airports and looks like a regular suitcase. Reassembling and disassembly are quick (20 minutes) and the bike, while twitchy, rides like a "normal" bike.
Like axototl, I usually find a place to leave the Bike Friday suitcase at a hotel or someone's house (friend or warmshower's host).
As I no longer camp, I have been able to reduce my gear to 2 panniers. Rear panniers are used with the Bike Friday.
Front Panniers and a large frame bag are used with the Waterford.
The Waterford handles well with just the front panniers. However, the rear panniers and the rack weights the Bike Friday heavily in the back and without enough weight upfront (in the converted rear rack bag), the bike has a tendency to "rear" up when pushing hard when starting or going uphill. While I have learned how to pack the Bike Friday to minimize it, I'm not sure with my set-up that I can eliminate it entirely.
I wrote
a review of the Bike Friday after I bought it and then did a tour on it. I wrote about my
10-years of experience with an S&S coupled touring bike.
I have found it to be much easier to take bicycles on trains as bicycles following whatever rules the train system has for bikes. I have a large bag that the Bike Friday can be put into and carried as large, unwieldy luggage. The few times that I've put the Bike Friday in a bag, it was very hard to carry (50 pounds slug over a shoulder) and I needed to stop and rest every few 100 feet (luckily, I knew this and rented a room next to the train station). In addition, the Bike Friday I have doesn't fold very well (the handlebar stem comes off and isn't hinged). If you get a folding bike that quickly folds into a small package (say a Brompton) that you can either roll or carry easily, then taking it on a train will be greatly eased.
If your plan is multi-modal travel (planes, high-speed trains) with a bike, I'm not sure a folding bike will fit your needs any better than a non-folding bike. My recommendation is to plan trips that minimize plane travel and use trains that take bikes. These trains often take longer but, for me, that is all part of the experience. In my travels, I've found that taking bike on ferries is usually without hassle.