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"Under locomotive series" (or "under locomotive series numbers")
Explanation: Without further context, I think that this straightforward translation is the right one. Could it be the heading of a way to sort different types of trains? One way to sort them would be under "locomotive series" or "locomotive series numbers"? Another instance where I came across the expression "locomotive series" was as a title for a series of paintings of locomotives. Again, this could be translated as "locomotive series of paintings", as a grouping under which the paintings could be sorted. Or maybe this refers to a keyword to find certain websites on "locomotive series"? You would find them "under locomotive series"
One thing for certain, "série de locomotive" is translated with "locomotive series".