Tender Locomotives 4-6-2 “Pacific”

The wheel arrangement 4-6-2 designates a steam locomotive that has a two-axle leading bogie, three coupled axles and a trailing axle. In different countries, this wheel arrangement is referred to as follows:

Pacific

UIC: 2'C1'

Whyte: 4-6-2

Switzerland: 3/6

France: 231

Turkey: 36

The main features of the Pacific were good guidance at speed due to the two-axle leading bogie/truck and a large firebox above the trailing axle that provided a high sustained steam production. So depending on country or operator, its evolution can be seen in different ways. Starting from a 4-6-0 “Ten-wheeler”, the 4-6-2 has the added trailing axle that could carry the deep, wide firebox compared to the smaller firebox of the 4-6-0 that had either been fitted between the frames or above the drivers. Compared to a 4-4-2 “Atlantic”, the 4-6-2 had an additional driving axle for additional adhesive weight and a bigger boiler. Finally, when compared to a 2-6-2 “Prairie”, the 4-6-2 had a two-axle leading truck for better running at speeds.

Although some locomotives were rebuilt into the wheel arrangement 4-6-2 in the last years of the 19^th century, the first locomotive to really implement the principles of this type was the class Q delivered in 1901 to the New Zealand Railways. It was meant to burn low-quality lignite and needed a wide Wootten firebox that could only be fitted above a trailing axle. It is not really clear if the name “Pacific” came from the fact that these locomotives had to travel over the Pacific, or just from a word play since the Pacific is an ocean that is larger than the Atlantic. Another explanation would be that the Missouri Pacific was the first railroad to use this type of locomotives in the USA and traditionally would have the right to give this wheel arrangement a name.

Within the first two decades of the 20^th century, the Pacific became the main type for heavy express traffic virtually all around the world with some exceptions. This was mainly driven by the switch from wooden to all-steel coaches and the sharply increasing passenger numbers in the years prior to World War I. With these locomotives, long and heavy trains could not only be accelerated to high speeds, but the powerful boilers also produced enough steam to sustain speeds of more than 60 mph or 100 km/h.

Britain was one exception where many railways still ordered 4-6-0 locomotives for express service in the forties since they didn't need a wide firebox due to their high-quality coal. There were also railways in several countries which ordered 4-6-2 locomotives with a narrow firebox. In the USA, Pacifics with axle loads of more than 30 tons like the PRR K4s reached an output of more than 3,000 hp. Due to the train weights in this country, other wheel arrangements like the 4-8-2, the 4-6-4 or the 4-8-4 had to be introduced in the twenties and thirties for heavier and trains or for use in the mountains. In total, US builders completed more than 7,000 Pacifics, most of these for domestic customers.

While most US Pacifics only had two cylinders, manufacturers of the European mainland built a considerable amount of four-cylinder compounds. The majority of these were used in France and the south of Germany. Although these required lots of working hours in maintenance, they were efficient on fuel and water. European manufacturers also exported many different types of Pacifics into South America, Africa, Asia and Oceania. This also included the British who took a long time to adopt the Pacific for their own needs. Many of the exported locomotives were Cape or metre gauge and did not meet the raw performance numbers of their standard gauge cousins, but enabled passenger trains on winding lines to achieve decent speeds.

Outside of the US, the Pacific enjoyed a long life and benefitted from improvements like feed water heaters, roller bearings, more efficient blast pipes or modern production processes. In Britain, the LNER was the first company to order a large number of Pacifics for long-distance trains. The most notable of these were both designed by Gresley: the A3, also known as “Flying Scotsman”, and the streamlined A4, setting the official world speed record for steam locomotives with 126 mph or 203 km/h. The Germans also built streamlined versions of their classes 01 and 03, but these later lost their streamlining. Germany was only one of many countries which used their Pacifics into the seventies, and other countries like India had to rely on this type of locomotives until the nineties.