RF&P was not quad tracked. It was always double tracked in the "modern" age. The figure of 518 track miles for the 118 mile railroad must have included all secondary tracks in Acca (Richmond) Yard, Alexandria Yard, the Dahlgren Branch, and all other secondary tracks throughout the RF&P.
If you watch closely while traveling on the RF&P, you can see evidence of numerous track relocations that were performed over the years. It's easier to see this in the winter, when the leaves are off the trees. A good example is the area between Aquia and Fredericksburg, where the old right-of-way can often be seen. By around the year 1900, the line had been double-tracked. I understand the civil engineers anticipated the eventual addition of a third track on the east side. More recently, several VRE stations have been built. You guessed it: VRE built the stations on the east side at places like Lorton, Woodbridge, Rippon, and Brooke. Of course, building them on the west side would have allowed construction of the third track in the location that was planned for it. But now that would possibly require the removal or relocation of those stations.
To be fair, I have to admit that there may have been good reasons for some of these decisions. But I have to wonder. And I have to wonder whether future decisions regarding this line will be made with any more care.
Tom