Mr. Duffy's words: "Existing train control systems in use today are much simpler and cannot ensure control of a complex mix of passenger and freight operations." I assume Mr. Duffy is alluding to "moving block" promised by PTC advocates. Trusting software to calculate braking distances of dissimilar length and weight trains is potentially dangerous. Mr. Duffy seems to be willing to exercise caution on this issue. A system which utilizes GPS offers certain advantages, however it is certainly no panacea. US railroads should be very sure that a centralized super computer system can actually and reliably replace existing wayside vital logic circuits.