how straight is straight[rails]?

I had a guy ask me if I can make a router within .001 straightness
over 48",
I am not sure this is possible with "normal" equipment.
Taken literally that would mean an accuracy of .00002 per inch!
{My ground straight edge is .002 per manf.}
Do the "big boy" CNC builders achieve this kind of tolerance?

Just to waste some time I checked my 5x8 in X.Y. best I can get
is +/- .005 over 6',using said straight edge.

The rest of his request was " flat to .005,and less than 8K"

Carl