The currently most powerful steam locomotives of the DFB, the HG 3/4, are at their limit. But those were the most powerful steam locomotives ever operating on this line, so there's no upgrade path... right?
Not quite. The Swiss Locomotive Works SLM built similar, but more powerful HG 4/4 locomotives for Vietnam. Unfortunately those were in scrap condition when the DFB finally found them. Among other things it turned out that the frames were broken, due to a construction flaw. They were brought back to Switzerland anyway. Because the original drawings could still be found, rebuilding many of the parts seemed possible. The new frames were sponsored by Stadler Rail, much of the other work is done by volunteers.
You might ask yourself: Why bring back scrap locomotives, if you basically end up building new ones anyway? Turns out that one major reason is that if you use *some* original parts, the whole locomotive is considered to be historic, and thus must only fulfill the safety requirements of the time the locomotive was originally constructed. A completely new locomotive would have to fulfill all of today's safety standards, which is not possible without major changes in the design.
It is amazing to see how enthusiasts build steam locomotives basically from scratch, sticking to the original drawings where possible, but making adjustments where necessary, and using the most modern technology, including CAD and 3D printing (the latter mostly to create casts).
A big Thanks to the DFB for showing us their workshop!