Did your relative have to have their farm or home lot surveyed either to establish boundaries with a neighbor or to have the property split up among several heirs? Those surveys and plats should be recorded in the same local records office that records deeds, mortgages, etc. (often the County Recorder in the United States). The surveys themselves may not provide major genealogical clues, but you may learn what caused seemingly arbitrary property lines (an old railroad in this case) or that a row of hedge was used by two brothers to mark their property line.
When accessing these records, make certain and determine how they are indexed–it is geographically or based on the landowners mentioned in the survey?