Thanks, everyone, for contributing! A few words about Rollwagen/transporters and Rollböcken/transfer trucks (?)/ from someone who has actually been working with them - well, not the trucks, just the cars:
Transporters have to be operated rather carefully; ours in Sweden, of the common European 8-wheel variety, had a 20 mph maximum on them (I remember reading about their being used on some 891 mm /2´11"/ ex-private line with just 30-pound rails, where the maximum was 15 mph!) But on a well-aligned track they ran quite OK, even with the 32-ton maximum load. Extremely heavy or long std. ga. freight cars could also be loaded onto TWO transporters. These were then coupled together with an extra-short coupling bar - the normal ones were from 20 to 25 ft. long. This was in fact the only REAL snag with the system, as you had to lift the !+*'#%! things by hand AND FIT THEM INTO the !!%%&?**!!#!"+**, very narrow link´n pin pockets. Connecting these was a two-man job, one man giving signals to the engine and the other fiddling with the bar - pooh. Disconnecting was easier... Switching a rake of empty transporters, however, was no problem - the system included a standardized, small kind of hook weighing perhaps ten pounds, for just rapidly linking them together.
Older transporters were unbraked but more modern ones had full air brake equipment, so the coupling bars had air connections with regular hoses. The bars consisted of either a single coarse pipe or a square girder of thin welded pipes, with the air running straight through these.
Transporter trains could be quite impressive - I have seen 15-car consists myself in the 1960s! If regular n.g. freight cars were included in the consist these were always coupled next to the engine. Obviously, EMPTY transporters had to be coupled to the rear end of the train...
Transporters were used on our single 2´7" line as well as on the 3´6" network, too. Obviously they helped a number of little lines to survive for an extra decade or two, but... Naturally "ramping on" or "off" the std. ga. cars took a lot of time via the special transfer ramps.
As std. ga. cars got bigger and bigger, a few 12-wheel transporters were also built. I think their capy. was 42 tons. Only on 2´11" gauge, not on the 3´6". (Bob N., did you see the 12-wheelers on the Montreux-Oberland...???!!! I think they have, or at least had, some 6-wheel transfer trucks also!!! To my knowledge these are still used quite extensively in Switzerland and Austria.)
None of these are in commercial use in Sweden any more, just on veteran steam & preserved lines, where I got acquainted with them - thank God I didn´t have to earn my daily bread working with them... However, I understand that accidents were rather uncommon, mainly due to the very slow speeds in switching. But the stories about what COULD happen to a transporter train are legion... One engineer told me he was coming with his big Tp class 2´11" diesel north of Stockholm on the freight for the papermill at Hallstavik one morning - and suddenly, as he looked back, he had an empty transporter in the middle of his train... (THAT time the ditched std. ga. gondola contained only old recycling paper...)
(In fact a few of the Rollböcken/trucks were in use in Sweden, in just one place, very locally, for ultra-slow hauls of half a mile or so, where std. ga. and 891 mm ga. met., well into the 1960s! These were designed according to the most common variety, having a kind of cradle for the std.ga. car axles. As to coupling the cars together I believe you could either use the regular s.g. couplers or loose c. bars between the trucks...)