trailer - look at the extrawheel trailer out of poland - I used this recently going over engineers pass in colorado, and have made the switch because i do a lot of dirt and off road touring.

the extrawheel uses a one bicycle front wheel, 26" or 700c - no need to carry smaller spare tire or tubes. much of the load is below the trailer axel, so pedaling standing up is VERY stable, unlike with panniers. the extrawheel voyager can be disassembled and packed in an airline legal box - not true for the extrawheel classic. I have brought one here to China, and on these STEEP cobblestone rutted country roads here in Yunnan provence, i believe its the way to go. The voyager has mounts for panniers, so if the trailer fails, just move the panniers to your rear rack. If your front wheel fails, move the panniers and trailer wheel to the bike and hide or store the trailer, go off to get another wheel. you never even know the thing is behind you until you feel it pushing somewhat while going down hills. I have ridden on pavement at 45 MPH with this trailer - something I never was totally comfortable doing on my trek 520 with panniers on it.

my new tour bike is optimized for trailer travel. the great thing about that is without the trailer, I have a bike that does not have traditional touring geometry so is VERY fun to ride.

the comment about train/bus travel is true, but not such a big deal. take off the yoke that connects it to the bike and bungie to the trailer, and the trailer is really quite easily loaded. weights on the order of 8 pounds without panniers.