An interesting development of the original transporter wagon concept (with bar couplers between each wagon) was that the bar couplers were discarded in favour of connecting all standard gauge wagons directly with each other by means of their ordinary buffing and draft gear. This was tried for a few years in Sweden just before the last narrow gauge freight lines were closed in the 1980s.
Special adaptors could be employed to couple a set of transporter wagons onto the end of an "ordinary" narrow gauge freight train. Continuous braking was no problem, either, as the train air line could be incorporated into the bar couplers, too.
Judging from early literature, the transporter wagon idea came about in Germany sometime around 1880 or 1890 (where in fact, later, Rollböcke were used a lot more than transporters). Transporter wagons with the unique Heberlein type friction brake system were in daily use in the old GDR (East Germany) well into the late 1980s.