I am not sure about the broken rail issue, but I can assure you that GPS accuracy is a big issue. Sure to atmospheric effects, which currently cannot be predicted with any degree of accuracy, GPS cannot be trusted in control system applications to be accurate enough. For example, the ionosphere, during periods of increased solar activity, will effect GPS signals in such a way as to signifgantly decrease accuracy. Even the military does not use GPS as a primary source out navigation data, when that data is being used for a control system, such as in UAV's (and they have the concept of "acceptable losses," something which is unacceptable for public transit. You all would be appalled to know how many UAV's crash sure to these GPS issues). GPS is an excellent secondary input to the control system, but campout be relied upon in a vital, fail safe system.there are too many unknown variables.