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Jean-Jacques Heilmann of France developed a new type of steam-electric railway locomotive, for which he took out a patent in 1890. The concept was novel, and the locomotives incorporated a number of advanced features.
1892-3 The first experimental locomotive was built. It was designed by Heilmann in collaboration with Charles Brown. It had a horizontally-opposed two-cylinder compound steam engine driving a DC generator. A small vertical engine drove a separate dynamo which supplied power for excitation and for carriage lighting.
There were two bogies, each with four axle-mounted armatures. Westinghouse air brakes were provided, and, unusually, these acted on brake drums rather than on the wheel rims.
The electrical equipment was supplied by Brown, Boveri et Cie. The six-pole dynamo was designed for minimum weight. It had a ring-wound armature, whose outboard bearing was housed in a three-armed bracket bolted to the stator casing. Its normal output was 600 HP at 400 rpm, and the maximum was 750 HP..
The engines, designed by Charles Brown, were built by the Forges et Chantiers de la Mediterranée, while the bogies were produced by the Compagnie française de matériel de chemin de fer (CFMCF), at Ivry. The main engine had one high pressure and one low pressure cylinder, with diameters of 425mm and 650mm respectively, the stroke being 300mm. The cranks for the HP and LP pistons were 180 degrees apart, so that the pistons moved in and out in unison. The engine was mounted transversely, and the design was constrained by the available width of the locomotive. In fact the cylinder covers emerged through the sides of the locomotive's body. Steam was admitted by semi-rotary Corliss type valves, worked by eccentrics. Steam flow was controlled by a double beat throttle valve.
Unsuperheated steam was supplied from a horizontal Lentz boiler with a corrugated firebox.
A crew of eight was required during the early trials, three of whom were occupied in keeping the main engine lubricated with oil and cooled with water.
Most of the above information is condensed from a detailed illustrated article in The Engineer.
Another locomotive (some sources say two) was built, improving on the initial design.
No. 8001 had a conventional Belpaire boiler supplying a Willans and Robinson high-speed vertical steam engine developing 1350 IHP at 400 rpm. The engine directly drove two DC dynamos. The exciter was driven by a small Willans engine developing 18 kW at 550 rpm. The electrical equipment was once again supplied by Brown Boveri .
The main engine was most unusual for the time, in having six cylinders. It was a compound engine, the 300 mm bore high pressure cylinder and the 480mm bore low pressure cylinder being arranged in tandem. The piston stroke was 400 mm. A separate crankshaft, gear-driven from the main crankshaft, operated the piston valves. This shaft also drove the governor.
The use of six cylinders was proposed by Mark Robinson of Willans & Robinson and by Natalis Mazen, of the Compagnie de l'Ouest.
Zoomable photograph of the Willans engine at the Willans and Robinson works at Rugby here. This shows the unusual construction of the crankcase, which was a riveted fabrication rather than the normal iron casting, presumably to save weight.
The locomotive made its initial test run on 12 November 1897. As for Heilmann's first locomotive, the entire weight, including coal and water, was available for adhesion, and the engine produced very little hammer blow. 
 Electric Railways: 1880-1990 by Michael C. Duffy