Locomotives with two, three and four cylinders have been commonplace. Six is unusual, but by no means unique; see the SNCF 160-A-1 Reheat Locomotive. Steam-motor locomotives tend to have a lot of small cylinders; for example the French high-pressure 232.P.1 had no less than eighteen. The record is probably held by the 1934 Bugatti proposal for a 2000HP locomotive with two 4-axle bogies, with each axle driven by an eight-cylinder single-acting steam motor, totalling an impressive 64 cylinders. This project was never built but one of the engines was, and duly tested.

At the other end of the scale, one-cylinder locomotives have been extremely rare, for excellent reasons. A machine with one cylinder on one side would be horribly unbalanced, and there would be the problem of the loco stopping on a dead centre. (ie with the crank at either the three o'clock or nine o'clock position, so that the piston thrust gave no rotational effect)

Nevertheless, here is such a locomotive. Built by Nielson (a name we have noted before in these pages) in 1857, It was a an 0-4-0 saddle tank with a single cylinder (10" x 16") mounted under the firebox and footplate, driving a horizontal equalising beam that moved in the slots seen at the rear.

A number of these were built for shunting in the collieries and ironworks of Scotland.

Left: The Nielson One-Cylinder Locomotive. The lower connecting rod is driven at 90 degrees to the top one. This was supposed to deal with the dead-centre problem, but I have my doubts... Wheel diameter 3' 2" 74 fire tubes 5' long.

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