Running an efficient railway has always relied on good regulation of trains, and, with limited scope for trains to overtake each other, it has also been preferred that the fastest passenger expresses be given priority over slower freights. Accordingly, in the days of steam, trains were classified by their importance, allowing signallers to award priority. To assist the signallers, trains would display their priority via a set of four lamps in an inverted T on the front of the locomotive, arranged in different patterns.
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