What is this large drag-grey piece? I see it being sold with 9v train tracks, and it is about the same size What is this piece?  I see it being sold with 9v train tracks
That's part 42936 Track System, Straight Track 16 x 8 x 2.



It was used in the Racers track system and was found in sets in the early 2000s such as 4588 Off-Road Race Track:



It really has nothing to do with 9V train track, but sellers don't always understand the details of different track systems and simply sell elements that look like track together.