SLM (Swiss Locomotive and Machinery Company Winterthur) initially delivered five locomotives with four coupled axles in 1924 for operation on the cogwheel line Krong Pha (later Song Pha) – Da Lat, the railroad company Compagnie Génerale de Colonies Paris in Indochina, presently Vietnam. These were the class HG 4/4, with road numbers 701-705. In 1929, the Esslingen Machinery Company built two locomotives with road numbers 706 and 707 as reparations due because Germany lost World War I. They were done from SLM plans. In 1930, came two more units from SLM with the road numbers 708 and 709. After World War II, not all nine cogwheel locomotives were still in existence and starting in 1947 the remaining units ran as road numbers VHX-40.302, 303, 304, 306, and 308 on the South Vietnamese State Railroad. Road numbers 304, 306, and 308 survived the Vietnam War in more or less operational condition and the last runs took place in 1975.
As part of the reactivation of the Furka Mountain Line, the two class HG 4/4 units, road numbers 40.304 (former 704) and 40.308 (former 708), were brought back to Switzerland in addition to four former class HG 3/4 units of the Furka Oberalp Railroad (FO). This was done in 1990 by the association "Furka Mountain Line Steam Railroad" (DFB) in an unprecedented promotion "Back to Switzerland". The rest of former road number 706 such as the frame with the wheelsets and the cogwheel drive with the cylinder blocks did not reach Switzerland until 1998 and then as a source of spare parts. Starting in 2006, the faithful restoration finally began of the two class HG 4/4 units, road numbers 704 and 708. This was done by qualified technical specialists at the DFB shops in Chur. The worn out frames had to be rebuilt utilizing still usable parts, which were finally available at the end of 2011. During the restoration, preference was given to road number 704 with the installation of the cylinder blocks as well as the new draft gear and buffer equipment (MGB/RhB norms), whereby the left high-pressure cylinder as well as the cylinder and slide valve cover had to be cast again. At the same time, the old riveted boiler was repaired extensively, while several parts had to be replaced on the firebox. The DFB shops in Uzwil, newly acquired in 2013, produced a new copper firebox as well as a new smoke box with a smoke box tube wall. The vapor collection box, backpressure brake housing, water tanks, cab, coalbunker, and most of the appliances were also newly built parts. Starting in July of 2018, work had progressed to the point that the first test and operation runs were done. Since the class HG 4/4 had never been used in Switzerland, first approval was required with the drafting of extensive calculations and safety verification. Finally, on October 16 came successful approval by the Federal Transportation Department (BAV) and on December 17, 2018 the DFB was able to receive final, unlimited operating approval from the BAV. The official maiden run took place on June 25, 2019 and it has enriched the DFB motive power since then. In the meantime, intensive work is being done in Uzwil on the sibling locomotive, road number 708, so that it can also be put into operation as soon as possible.