Here's a couple of examples.
Next to the Mile marker. The sign says "BEGIN" Test Mile on one side and "END" Test Mile on the other.

On the Conrail side of things it says "Begin Measured Mile" on one and usually they have a fluorescent marker on the post. The reciprocal says "End Measured Mile" as shown below.
Testmile29.JPG
Hopefully this helps.
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