What is this black brick with red tubes inside?
This is a 7049b - Wheel brick. It was used with the first LEGO wheels back in the 60's to create vehicles like this:



This was a very common piece (in white and black) when it was produced between 1959-1992 and was used in 176 sets according to BrickLink.

Per @GalacticCowboy's comment: As shown in the inset, the wheels had a metallic axle that snapped into the holes. Because they were modular, you could use wheels in whatever configuration you desired.