According to the British historian of science M. J. T. Lewis, the Diolkos represented a railway, in the basic sense of a prepared track which so guides the vehicles running on it that they cannot leave the track.[3] Measuring between 6 and 8.5 km (3.7 and 5.3 mi),[43] and being open to all on payment, it constituted even a public railway, a concept which according to Lewis did not reoccur until c. 1800.[5] Also, its average gauge of around 160 cm (5 ft 3 in)[32] is similar to modern standards.
However, a close examination of the excavated tracks may give a different picture. While there is agreement that the grooves in the eastern part were cut deliberately into the stone slabs to guide cart wheels,[44] those in the western section are interpreted by some authors as a result of wear or do not appear at all.[45] On the other hand, the marked cambers of this road section may point at deliberate tracks as well.[32] Generally, varying forms of the grooves can also be explained by the long period of operation of the Diolkos, during which modifications and repairs must have significantly changed the appearance of the trackway.[46]"
So, was the Diolkos the ancient predecessor to the modern railway?
Perhaps not, but certainly the idea of cutting standard-spaced continuous parallel grooves in rock with the approximate spacing of modern day railroad tracks (probably to keep load-bearing wooden wheels of some sort, in place) seems to be one of the forerunner early historical ideas which (when coupled with subsequent ideas, i.e., metal tracks, steam-driven machines to do the pulling, etc., etc.) lead up to modern railroads, although, admittedly there's around a two millennia (give or take!) gap between this idea and the next railroad-related one! :-)