What's with the differing Unit Numbers ? 4119 Loco Number , 56 Train Number ?
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UP, SP, and maybe other western railroads used the number boards to display the time-table train number for positive identification of the train, regardless of lead engine number. This allowed them to put a helper on the head end, move the numbers to the helper, and proceed. (With train orders, a train was identified by the number of the lead engine, making such operations difficult for the dispatcher.) For extras, X or X- and the engine number was placed in the number boards. For sections, 1- for first section, 2- for second section, etc., was the way to identify each section. There were many advantages to this system--lead engine could be swapped out without having to change train orders, sections could be swapped if a leading section broke down, etc. There were probably other reasons I am not aware of. Find a recent photo of UP 4014 and you will see X4014 in its number boards.
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