In the beginning, most locomotives had only one or two axles which were powered. In total they had three or four axles, but the other axles were only used to carry additional weight or to guide the locomotive at speed and in curves. Although there were several locomotives with three or four coupled axles in the first years, they did not become a standard. But after the middle of the 19th century, developments required changes. On the one hand, the mass of the goods to be transported grew and one tried to combine these into a fewer number of longer trains. Additionally, many companies started to build railway lines in the mountains, where huge amounts of raw materials could be obtained.
The first step were six-coupled locomotives which had no leading or trailing axles. They could carry the same size boiler as a 2-4-0 or 2-2-2, but use all their weight for traction. Most countries around the world only used these in freight service or as shunters, but some countries had carefully laid and maintained tracks which also allowed to use 0-6-0 locomotives in passenger service.
As both freight and passenger trains became heavier and faster at the same time, six-coupled locomotives had to be equipped with leading axles to carry a heavier boiler and to improve the running characteristics at higher speeds. On the one hand, this resulted in the 2-6-0 that was most frequently used for faster freight trains or mixed traffic. The other result was the 4-6-0, that could carry an even heavier boiler and had better guidance at high speeds. Their main task was heavy passenger traffic, but there were also many 4-6-0 designs for heavy and/or fast freight traffic and for express traffic.
Around the turn of the century, there was a general trend to equip tender locomotives with a trailing axle to allow for a large, wide firebox that was mounted behind the last driving axle. This was not only the case for the 4-4-2 “Atlantic” express locomotive, but also for the 4-6-2 “Pacific” that could haul even heavier express trains at high speed. In parallel to the 2-6-0, the 2-6-2 was created as its equivalent with a larger firebox. Later, even the wheel arrangement 4-6-4 with a two-axle trailing truck was created.
As train weights rose again, six-coupled locomotives were not powerful enough anymore in some cases. This happened at first for the all-adhesion freight locomotives, what led to the 0-8-0 and later to the 2-8-0. On the steep ramps of the Alps, some operators generally switched to eight-coupled locomotives in the 1870s. For passenger service on level lines or with less steep inclines, locomotives with three coupled axles managed to fulfill the needs until the end of steam in most cases.