A southbound coal train is rolling along on an Approach signal, running behind a local freight that's just ahead. This is a 3.7 mile stretch of dual main (reverse signaled), so the left signal is for a northbound, which is just moments away (these signals are approach-lit, so the train is on the circuit). Under CSX signal rules, this is an Approach Medium, which means the train should approach the next signal (at Dudley, 2.5 miles ahead, the end of this stretch of double track) at no more than Medium speed (30 MPH under CSX rules). This is known as a "speed" signal. On NS, this same signal indication would be Approach Diverging. That would be a "route" signal, and it says the train should approach the next signal expecting to take the diverging route no more than the maximum as prescribed in the timetable special instructions (usually that would be 45 MPH in most CTC territory). In either case--route or speed--the signal indications give the engineer clear knowledge of what to expect at the next signal, and how to operate his/her train safely.