If they observed the rules in effect on the "steam" railroads of the time, two white lights on the head end would denote an extra train (not in the timetable), and two green lights indicated that there was another section following. I believe the IT was largely a timetable-and-train-order operation, making such indications essential. For example, if Train No.21 had orders to take siding to meet Train No. 32 at suchandsuch siding and No. 32 was displaying green signals it meant that this isn't all there is of Train 32 -- wait for the rest of it before proceeding. (Two white lights would mean this isn't Train 32 at all -- you haven't met it yet.)
