The major rail lines in German East Africa were the Usambara and Tanganjika lines, both of which ran on metre gauge (i.e. 100cm narrow gauge). I assume the first picture is one of those lines, judging from the size of the wagons.
The closest option might be using S gauge (22.4mm wide) tracks, which would be about right for a vehicle scale of 1/50, and scratch-build the vehicles. Alternatively, you could use the S gauge locos and rolling stock, but given that it is pretty much an American thing, it will a) be had to get outside the US and b) probably won't include German designs.
Alternatively, you could go for one of the various narrow gauge small railways. In that case, regular 00 gauge tracks would correspond to about 750mm (a common German narrow gauge), although you would probably have to scratchbuild "authentic" locos and rolling stock if you cannot find them in the 0n30 market.