One of the best known of electrical freaks--the Heilmann locomotive (Fig. 14)--is a very good example of the way in which an invention may be tried with enthusiasm, rejected with contumely, and revived at a much later date in an improved and more promising form. The Heilmann locomotive was practically a generating station on wheels. It carried a boiler and engines, which drove a dynamo, the current from which was led through controllers to motors coupled to the wheel axles. It was an enormous affair, over 18 metres long and running on sixteen wheels; extensive trials were made with it on the Western Railway of France in the early nineties. Some advantage was gained in smoothness of running, ease and uniformity of control, and improved acceleration; but its great weight, cost, and complexity were against it. In spite of the cordial support given to it by railway engineers, it was soon relegated to the scrap-heap. [Illustration: Fig. 15. Electro-turbo-locomotive built by the North British Locomotive Company for experimental purposes. This locomotive is a 'generating station on wheels.' It carries a steam turbine driving a dynamo which supplies current through a controller to motors geared to the axles.] The Heilmann locomotive, it will be noticed, is similar in principle to the petrol-electric systems of propulsion now in use for road traction. But it is probable that the idea would never have been heard of again in connection with railway work had it not been for the appearance of the steam turbine. It was natural that the locomotive engineer should consider how the turbine could be applied to his purposes; and the first step in this inquiry made it plain that some electric method of control was necessary between the high-speed turbine and the driving axle. Consequently, when the engineers of the North British Locomotive Company set to work in 1909 to design an 'electric turbo-locomotive,' they produced something not at all unlike the Heilmann locomotive. The equipment consists of a steam turbine, with elaborate condensing plant, a generator, and a group of driving motors (Fig. 15). The turbine runs at 3000 revolutions per minute and drives a continuous-current dynamo, the current from which passes through controllers to four motors which can be run in series, or two in series and two in parallel, or all in parallel, according to the draw-bar pull required. Trials with this locomotive were begun early in 1910, but it is yet too early to say whether it will be more fortunate than the Heilmann locomotive, and whether it is likely to delay the advance of the electric locomotive proper, fed with power by overhead wires from a central power station. [Illustration: Fig. 16. Diagrammatic sections of the Behr electric mono-rail car. The car is balanced on the summit of a continuous trestle and is designed for speeds up to 120 miles per hour.]