Overview
The United States Army Transportation Corps (USATC) S100 Class is an 0-6-0T (No leading or trialing wheels, 6 driving wheels, T for Tank engine) steam locomotive that was designed for switching (shunting) duties in Europe and North Africa during World War II. Several were later used on railways in Austria, Great Britain, France, Greece, Italy, Yugoslavia, Palestine, Iraq, Iran, Israel and China. There are many surviving today in Europe, and some in use on Heritage railways.

Wartime Development
The S100 is a side tank design by Col. Howard G. Hill. In 1942, the USATC ordered 382 S100s from Davenport Locomotive Works of Iowa, H. K. Porter, Inc, of Pittsburgh and Vulcan Iron Works of Wilkes-Barre. They were shipped to Great Britain in 1943 where they were stored until 1944. After D-Day, they were shipped to Continental Europe.
They could not just ship any locomotive over to Europe. A standard American 0-6-0T could not run on a European railroad because our loading gauge is larger than Europe's. Even though we have the same track gauge (4 feet, 8.5 inches), they would be too big for stations and tunnels. So they had to design a locomotive that could be built in the US and shipped to Europe
After the War
After the Second World War, SNCF bought 77 S100's and designated them class 030TU. Jugoslovenske železnice (Yugoslav State Railways) bought many S100's and designated them class 62. In the 1950s JŽ assembled more examples bringing the number of class 62 to 129. The Hellenic State Railways in Greece acquired 20 S100's and designated them class Δα (Delta-alpha). Österreichische Bundesbahnen in Austria acquired 10 and designated them class 989. Ferrovie dello Stato in Italy acquired four and designated them class 831.

The Oranje-Nassau Mijnen, a coal mining company in The Netherlands acquired two S100's (USATC 4389 and 1948)and numbered them ON-26 (Davenport 2533) and ON-27 (Davenport 2513) respectively. The ON-26 survived the scrapyard and was sold to the museum railway Stoomtrein Goes-Borsele.

The Southern Railway (GB) bought 15 S100's (14 for operational use and one for spare parts) and designated them USA Class. Other S100's entered British industrial use with the National Coal Board, Longmoor Military Railway, Austin Motor Company and others.

China acquired about 20 S100's, designating them class XK2. In 1946, Egyptian State Railways bought eight and numbered them 1151–1158. The UK War Department loaned six to Palestine Railways. In 1946 PR bought two of these, both of which subsequently entered the stock of Israel Railways in 1948.

Iraqi State Railways bought five, designated them Class SA, and gave them fleet numbers 1211-1215. All five were Davenport-built examples. At least two were still in service in March 1967: 1211 at Basrah and 1214 as the station pilot at Baghdad West.

Post war design influence
Several European railways produced designs based on the S100. JŽ added to their Class 62 by ordering several similar examples from Đuro Đaković (factory) of Slavonski Brod, Croatia. These differed in minor details, principally the use of plate frames instead of bar frames, resulting in a higher boiler pitch. This gives the steam pipes a shoulder instead of being straight, and requires smaller domes with a flatter top to fit JŽ's loading gauge.

Poland built several TKh Ferrum locomotives. These have a similar outline but include various differences such as the use of 2 domes instead of 3, driving onto the second axle instead of the third, a different cab, etc.

The British Great Western Railway (GWR) had used many S100s in South Wales during the Second World War. The GWR 1500 Class was partially inspired by the S100 in its use of outside cylinders and short wheelbase.

Continuing Commercial Use
A small number of former JŽ 62's remain in commercial service, more than 65 years after they were built. At least two work as switchers (shunters) at the ArcelorMittal steel plant in Zenica, Bosnia-Hercegovina

Preservation
More than 100 S100s survive: either preserved, stored or derelict. Most are in Europe or North America but there are also two in China and one in Egypt. Project 62 has an online database of them

Crucial Information
Power type Steam
Designer Howard G. Hill
Builder Davenport Locomotive Works (109), H. K. Porter, Inc (150), Vulcan Iron Works (123)
Build date 1942–1944
Total produced 382
Specifications
Configuration 0-6-0T
UIC classification C nt
Gauge 4 ft 8 1⁄2 in (1,435 mm)
Driver diameter 4 ft 6 in (1,372 mm)
Note: These are VERY large driving wheels. Most switch locomotives have driving wheels of 3 feet or less, for better traction.
Minimum curve 150 ft (45.72 m) radius
Length 29 ft 6 in (8.99 m)
Width 9 ft 0 in (2.74 m)
Height 12 ft 1 5⁄8 in (3.70 m)
Locomotive weight 100,650 lb (45.7 tonnes)
Fuel type Coal or Oil
Fuel capacity 2,500 lb (1.1 tonnes) coal, or 300 US gallons (1,100 l; 250 imp gal) oil
Water capacity 1,200 US gallons (4,500 l; 1,000 imp gal)
Boiler pressure 210 lbf/in2 (1.45 MPa)
Firegrate area First 50: 19.4 sq ft (1.80 m2),
Remainder: 18.3 sq ft (1.70 m2)
Heating surface:
– Tubes 790 sq ft (73 m2)
– Firebox 86 sq ft (8.0 m2)
– Total 876 sq ft (81.4 m2)
Superheater type None
Cylinders Two, outside
Cylinder size 16.5 in × 24 in (419 mm × 610 mm)
Valve gear Outside Walschaerts
Valve type 8-inch (203 mm) piston valves
Performance figures
Tractive effort 21,630 lbf (96.2 kN) (how much force they can exert on the rails)
Factor of adhesion (how strong a grip on the rails, IIRC) 4.65
Career
Operator(s) USATC

For modelers out there, Bachmann Trains makes a very nice HO scale (1:87) model of an S100, decorated in USATC markings, as well as a quarry company, among others.