0-4-4-0

In the electric and diesel eras, the Bo-Bo is comparable and closest to the Meyer arrangement of two swivelling bogies.

[citation needed] Although rigid duplex locomotives were also constructed with pairs of driving axles and the 0-4-4-0 driven arrangement, these were intended for express passenger service and so were given 4-4-4-4 overall arrangements with leading and trailing bogies for stability.

This required the development of new techniques in locomotive design for which trials were held and the entrants used various forms of articulation in order to place the most useful power through their driving wheels.

[3] Thouvenot in 1863 produced an 0-6+6-0T design which was closer to the Fairlie type, with a double boiler and swivelling bogies.

[2][4] The first Meyer locomotive, L'Avenir, was built in 1861 and was also derived from a Semmering Trials design, the Neustadt [de].

Meyer used compounding at first and so the cylinders were placed at the inner end of the bogies, where the intermediate pressure pipework between them could be kept shorter.

A decade after Seraing, Robert Fairlie revived the concept and was granted a patent for his design in 1864.

[2] Placing the cylinders at the outer ends of the power bogies left a space between them and allowed depth for a conventional firebox, grate and ashpan.

[12] The Mallet type developed and outgrew the original wheel arrangement, particularly in the USA, also gaining tenders and often being simple expansion engines rather than compounds.

From 1911, Italian State Railways built at least twelve 950 mm gauge Mallets as the FS R.440 [it] class.