For any two points, there is at least one constant-direction track leading from one to the other. (Constant direction relative to true north, anyway, and maybe constant magnetic as well.) Given the lat-lons of the two points, it will take you a couple minutes to calculate the direction of that line-- is that what you mean by "the rhumb line heading of the track at the departure point"?
In other words, which "track" are you asking about?
(You know about straight lines on Mercator charts being rhumb lines?)