The Siegfried is an Imperial Guard light tank first developed during the civil war on Krieg. It was seen as a replacement for the Sentinel, whose locomotive system was too complex and armoring too light for the dangerous conditions on Krieg at the time. Derived from the famous Land Crawler agricultural vehicle, it was armed with a Multi-Laser and replaced the Land Crawler's simple yet sturdy catalytic engine for a more powerful gas turbine one which, despite being fit with considerably more armor, doubled the Siegfried's top speed. To put things into perspective, the Land Crawler is a over glorified Imperial farming tractor, yet the Kriegers still found some use for this vehicle and modified the tractor into a tank.
However, as much as the Siegfried improved on the Land Crawler, it was still no Ragnarok. When Krieg's lovely civil war ended, the return of Sentinels kind of made the Siegfried redundant. Even if the Siegfried fared better in the trenches of Krieg than the chicken walker, the Sentinel is still faster and more agile than the Siegfried, and the Sentinel can traverse terrain that the tractor tank would find impassable. Any decent anti-tank weapons such as Lascannons can also turn the Siegfried inside out, but to be fair, they'd do the same to the Sentinel, so that doesn't really count.
Despite all this, the Siegfried still finds some use as a mobile reserve and breakthrough tank, and probably serves as a popular Fast Attack option for PDF forces that don't have access to Sentinels.
Another 40k scale Siegfried, with a little more effort put into it, seems to converted from a model of a french Renault UE Chenillette with a super structure built on top.