R uses several forms of signalling. In IR manuals reference is made usually only to 4 main types of systems, Lower Quadrant semaphore, Modified Lower Quadrant semaphore, Multiple Aspect Upper Quadrant semaphore, and multiple-aspect colour-light signalling. But in practice there are some variations in the kinds of colour-light signalling seen, so for ease of analysis, the following classification is used here. (Abbreviations in parentheses given for ease of reference in the text that follows.)
Two-aspect Lower Quadrant semaphore signalling (2LQ) Modified Lower Quadrant semaphore signalling (MLQ) Multiple Aspect Upper Quadrant semaphore signalling (MAUQ) Two-aspect Colour-Light signalling (2CL) Three-aspect Colour-Light signalling (3CL) Four-aspect Colour-Light signalling, normally known just as Multiple Aspect Colour-Light signalling (MACL) These are explained in detail in the section on aspects and indications of signals.
In addition to these, there are some in-cab warning systems (AWS), and of course flag/lamp/hand signals for emergency use.
Source - IFRCA.org
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