Articulated locomotive

Articulation allows the operation of locomotives that would otherwise be too large to negotiate a railroad's curves, whether mainlines or special lines with extreme curvature such as logging, industrial, or mountain railways.

The smaller pair of cylinders near the cab was fed with high pressure steam directly from the boiler and then the steam was passed into a pair of low-pressure cylinders at the front, with larger diameter to offset the lower pressure, before exhausting through the smokestack.

While the thermal efficiency was greatly improved through the compound use of steam in Mallet designs, the large low-pressure cylinders posed unique limitations, both in terms of loading gauge (the cylinders could only be as large as the track and track-side infrastructure allowed) and in terms of performance at speed.

The large and consequently heavier pistons caused stability issues at higher speed, which generally limited compound expansion articulated locomotives to below 30 or 40 miles per hour.

A notable exception to this was to be found in later iterations of Norfolk & Western Y-class 2-8-8-2s, which could and did often exceed 50 miles per hour in service as well as being one of the hardest-pulling steam locomotives ever built.