He knew what he was talking about, but I'm pretty sure others knew that before he did.
For example, from wiki's info on "gravity train": "...In the 17th century, British scientist Robert Hooke presented the idea of an object accelerating inside a planet in a letter to Isaac Newton. A gravity train project was seriously presented to the Paris Academy of Sciences in the 19th century. The idea was rediscovered in the 1960s when physicist Paul Cooper published a paper in the American Journal of Physics suggesting that gravity trains be considered for a future transportation project..." Several of these concepts worked out the 42min time from the fact that the time only depends on the density of the earth and the gravitational constant. E.g.:
http://www.math.purdue.edu/~eremenko/dvi/gravsol.pdf