How many years after the start of construction were trains running along the originally planned route?

Not answerable because it does not state which railway, but construction times obviously depended very heavily on the nature of the work involved, the distance covered, the size of workforce available, and accessibility at intermediate points for supplying materials, labour, food for the builders, etc. Laying a 100 miles of single-track railway across a plain in temperate conditions, for example, would be quicker and easier than covering the same distance in rugged, mountainous country beset with fairly extreme weather.