Rigid-framed electric locomotives were some of the first generations of electric locomotive design. When these began the traction motors of these early locomotives, particularly with AC motors, were too large and heavy to be mounted directly to the axles and so were carried on the frame. One of the initial simplest wheel arrangements for a mainline electric locomotive, from around 1900, was the 1′C1′ arrangement, in UIC classification.

Some of these locomotives had their driving wheels coupled with coupling rods, as for steam locomotives. Others had individual motors for each axle, as would later become universal.

By the middle of the century, the bogie arrangement for locomotives became more popular and rigid-framed locomotives are now rare, except for small shunters.

1′C1′

1′C1′ is the UIC classification for a railway locomotive with a wheel arrangement of three coupled driving wheels, with a leading and trailing articulated pony truck. The driving wheels are coupled by external coupling rods.

The description is most commonly applied to some locomotives from the early decades of electric locomotive design, when the traction motor or motors were mounted on a rigid frame and drove all the driving wheels coupled together. To permit higher speeds for fast passenger services, leading pony trucks were added. An advantage of electric locomotives was that they could easily have two driver's cabs, so avoiding the need to turn them at terminus stations and so the wheel arrangement was made symmetrical to run equally well in both directions.

Steam locomotives

The equivalent is 2-6-2 or 2-6-2T Prairie arrangement for steam locomotives was a popular one for a long period. The first was the South African Cape Government Railways 2nd Class 2-6-2TT of 1875. Building of 2-6-2T classes, such as the GWR Prairies or Standard Class 2s, continued almost to the end of steam.

Many of these prairies were tank engines, which were able to run equally well in either direction. This was aided by the symmetry of the wheel arrangement. These were often used for suburban passenger services, which needed fast turnarounds at city terminus stations, without needing to turn the locomotive on a turntable. Although quite powerful locomotives were used, to give good acceleration between closely spaced stations, they did not need the sustained speed or fuel capacity of an express locomotive.

Electric locomotives Italy

This first 1′C1′ arrangement was only a common one for electric locomotives in the early years. It was used for some early AC examples, mostly Italian, between 1900 and 1920. Italian railways had begun electrification with a low-frequency three-phase AC system, to the designs of the Hungarian Kálmán Kandó. These used large, 2 metre diameter, motors, powerful enough that only one or two motors were needed for the locomotive, but also too large diameter to be mounted easily for a drive to the axle. Accordingly, Kandó frame mounted the motors on resilient springs and used coupling rod drives to the wheels.

The first of these locomotives were the Italian RA 361, later FS class E.360, for the 1902 electrification of the Valtellina line.

For these early small locomotives, the power was sufficiently low that both motors could be coupled to the crankpin of the centre axle with a simple rigid triangular coupling rod. Secondary rods carried the drive to the other wheels. The motors were rigidly mounted to the frames and so the centre crankpin had a vertical slide mechanism in the triangular rod, for suspension travel. Each of these two motors were rated at 600 bhp and weighed 8.2 metric tons, 40% of the total weight of the locomotive.

France

The 1′C1′ arrangement was considered as something of a standard pattern for small passenger locomotives, so that when the French Chemins de fer du Midi electrified at 12 kV Hz AC in 1912, six locomotives of this type were ordered for trials, from the major electric locomotive makers of Europe: E 3001, E 3101, E 3201, E 3301, E 3401, E 3501. Only E 3201, built by the American Westinghouse, was considered successful and it lasted in service under the SNCF as 1C1 3900 until 1959. E 3401, by Jeumont, used the 1′Co1′ layout, with separate motors for each axle.

When the production locomotives from these prototypes were to be built, they were to follow the Westinghouse design of E 3201 but they were so delayed by the war that when they emerged as 2C2 3100 they had become a 2′C2′ design with bogies instead of pony trucks and with three vertically mounted motors by Dick, Kerr of Britain, not Westinghouse.

Sweden

Some of the last 1′C1′ locomotives in service were the Swedish SJ D and Da classes. The D class were built from 1925 to 1943 in several sub-variants and a modernised version, the Da, in the mid 1950s. Both remained in service until around 1990. Both had a pair of traction motors, up to for the Da, and retained the coupling rod drive. The motors were geared to a single jackshaft placed level with the other axles, in the position where a third equi-spaced axle would have been, and so did not require the complicated or heavy triangular drives of earlier designs.

Diesel locomotives 1′C1′ was not generally used for diesel locomotives. The overlap between viable diesel locomotives, owing to the development of the first high-speed diesel engines from around 1930, and the era of small rigid framed driving wheels was short. The first diesel shunting locomotives were of C or 0-6-0 wheel arrangement, as they were neither heavy enough nor fast enough to need the pony trucks. These low-powered shunting locomotives could also make do with a single traction motor.

More powerful mainline locomotives used multiple traction motors, one per axle. This was mostly due to the locomotive's generator being able to produce DC current, thus allowing simpler and more controllable DC motors to be used. These could easily be made in the smaller diameters needed for per-axle use, rather than the large diameter many-pole AC motors. As diesel locomotives developed, the per-axle traction motor design became ubiquitous, with 1′Co1′ and 1′Do1′ designs in early years, later with bogies as Bo′Bo′, then Co′Co′ and heavier designs.

A rare exception was the Armstrong Whitworth hydrostatic drive locomotive of 1929 for the Buenos Aires Great Southern Railway of Argentina. This used a single transmission and jackshaft drive from one end, with coupling rods. The locomotive was unsuccessful, owing to problems with the transmission and lay unused for several years before being scrapped in 1943.

1′Co1′ The 1′Co1′ wheel arrangement was similar, except that the wheels were driven by separate traction motors for each axle, rather than them being coupled together. As each axle was driven, there was no longer a need for the coupling rod connection between them. This independence would in time raise the issue of wheelslip, starting with a single axle, but that was not an issue for the early locomotives, with their low ratios of power to adhesive weight.

Diesel-electric locomotives used the 1′Co1′ arrangement with separate traction motors, rather than the coupled 1′C1′. This was because these were some decades later than the first electrics, by which time traction motor technology had developed. Also the generators on board the locomotive could generate DC, rather than AC. Control of DC was more sophisticated by this time and there was no need for the traction motors to be the large diameter that the 1900 AC electrics had required.

The first British mainline diesel locomotive, Armstrong Whitworth's 800 bhp locomotive built for the LNER in 1933, was of this type. This locomotive was unsuccessful and scrapped after only a few years, in 1937.

2′C2′

In 1914 Italian State Railways constructed two series, the E.331 and E.332, of three-phase AC locomotives. These were intended to be a development of the 1′C1′ E.330, suitable for lighter secondary lines. The heavy central V coupling rod and closely set paired motors of the E.330 were replaced by two motors set either side of the transformer and control gear, and with a lighter rod drive in order to reduce Hammer blow, using lighter rods and a pair of jackshafts set beyond the coupled wheels. Placing the jackshafts at axle level reduced the articulation angles needed in the coupling rods, so avoiding the mass of the older triangular rods, or the need for vertically slotted crank bearings. Overall, this reduction in reciprocating mass made the overall locomotive heavier and with greater overhang at the ends, requiring bogies rather than pony trucks.

2′Co2′

After the initial promise of electrification on the Shildon–Newport line coal traffic, the North Eastern Railway commissioned a large express passenger locomotive, later classed as EE1. Although the mainline electrification through York never took place, the locomotive itself was a success in its brief trials. The driving wheels were unusually large, following the steam locomotive practice with which Darlington Works were more familiar, and the electrical equipment was supplied by Metropolitan-Vickers of Manchester. The locomotive was heavy for a DC electric and in consideration of its high design speed, four-wheel bogies were used at each end.

(1′Co1′)+(1′Co1′)

The American New Haven EP-2 of 1919–1927 combined two 1′Co1′ sub-frames beneath a single boxcab body to make a (1′Co1′)+(1′Co1′) locomotive. As the two frames were articulated together and the body on top was not a structural component, this was a (1′Co1′)+(1′Co1′) rather than (1′Co1′)(1′Co1′); i.e. articulated in two units, rather than with bogies beneath a single structure. At 2,000 hp, they were around the same power as a pair of the previous EP-1s, which had often worked in multiple, although also lighter and more stable at speed.

The EP-1s had required conversion from a Bo'Bo' arrangement to (1′Bo)(Bo1′), in order to prevent oscillation or 'nosing' when running at speed. The same use of pony trucks was designed into the EP-2 from the outset. The central carrying axles were largely pointless though, having no guiding function. When additional locomotives, the EP-3 and then EP-4, were required in the 1930s, these used a (2′C)+(C2′) arrangement, with the carrying axles moved to the ends as twin-axle bogies.

1′B1′

The smaller four-driver 1′B1′ layout was not generally used, but the Swiss narrow-gauge Rhaetian Railway did have one class of seven, the Ge 2/4 in 1912. These were low-frequency AC locomotives with a single repulsion motor mounted in the body, driving by rods downwards to a central jackshaft and then to the driving wheels.

1′D1′ and 1′E1′

Larger and heavier locomotives required more driving wheels, in order to provide enough adhesion whilst limiting axle load. Some earlier locomotives had already had four driven axles, but these had used either the Bo+Bo articulated arrangement or Bo′Bo′ with separate traction motors for each axle and two bogies.

The first electric locos to extend the 1′C1′ arrangement to 1′D1′ were the Swiss Rhaetian Railway Ge 4/6 class of 1912. In the same year, the Bern–Lötschberg–Simplon railway's used a 1′E1′ arrangement, with five driven axles, two traction motors of each and a rigid triangle rod drive to the centre axle. At the time, these were the world's most powerful electric locomotives. Each motor weighed 14 tons and was supplied by a separate transformer with a 12-stage tap changer. The taps for each motor were changed alternately, giving an effective 24 step control. The Swiss railways used low-frequency single-phase AC, at 11 kV for the Rhaetian and 15 kV for the BLS.

By 1922, the Italian three-phase system was also using heavier and more powerful 1′D1′ locomotives, such as the FS class E.431. These now had a wheelbase so long that a Zara bogie was used to articulate them.

1′Do1′