If you web search SEPO/POSE railroad.net you will get a topic page from railroad.net forums that discusses all of the info on these trains in great detail.
Not sure of the context in which you're seeing it used, so this is just a guess. Pan Am Railways (formerly Guilford, and before that Boston & Maine/Maine Central) at one time used four-letter symbols for some of their regularly-scheduled trains. I'm a bit out of touch, so I'm not sure if they still do. POSE stood for Portland, Maine to Selkirk, NY, and obviously SEPO was the return train.
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