Compound Systems
There are many compound systems and configurations, but we can define two basic types, according to how HP and LP piston strokes are phased and hence whether the HP exhaust is able to pass directly from HP to LP (Woolf compounds) or whether pressure fluctuation necessitates an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver (receiver compounds).
The eternal problem with compounds is starting: for all cylinders to take their weight, it is advisable to have some way of short-circuiting the HP cylinders and getting steam at a reduced pressure directly to the LP cylinder(s); hence many of the patented compound systems are associated with particular starting arrangements. The de Glehn 4-cylinder system is probably still the most sophisticated of all with independent HP & LP cut-off and a rotary valve, called a lanterne allowing independent working or combinations of HP and LP groups. Most other systems employ starting valves of various kinds. Another criterion is whether the valve gears of the two groups are wholly independent or linked together in some way.
Read more about this topic: Compound Locomotive
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