One of the big problems with rolling out infrastructural upgrades to the US train network is, in general, its size. Consider that there are still substantial se

One of the big problems with rolling out infrastructural upgrades to the US train network is, in general, its size. Consider that there are still substantial sections of freight rail in the country that are what is referred to as "dark territory," meaning that there is no signally installed. Additionally, there is usually no radio or cellular coverage in these areas. Train motion in these areas is typically governed via track warrants, meaning that an engineer must obtain written permission from a dispatcher before entering the area (and only one train is given this permission at a time). In the NE corridor, everything is of course fully signalled, but installing additional wayside equipment such as for PTC is a huge project over many miles of rail, even in relatively small areas. Solar-powered radio equipment has driven down the cost of installation, but not by as much as you might think. A big part of the high cost is the engineering requirements of the federal railway administration, which are (for good reasons) extremely strict and require proven high reliability. This reliability must apply everywhere. GPS availability can be iffy under a number of circumstances, most obviously tunnels. There needs to be some non-GPS method of positioning trains in most systems. An example which the HN community will be relatively familiar with is the BART, which features automated train control. BART trains position themselves (and receive speedcodes for central control) via a continuous antenna strip mounted to the safety cover of the third rail. If you inspect the third rail across from a platform you will see it, as well as occasional cables connecting to the trackside control equipment. Installing this sort of thing over miles gets very costly! The closest analogy to what you are describing is a product called GE Trip Optimizer that uses information about the route and train consist to automatically control throttle for fuel saving purposes. Trip Optimizer does not meet requirements for a safety system, though, and so the engineer is always permitted to override it (and must if any unusual circumstances are encountered), and the engineer must manually account for signals. To be a full PTC system it would still need quite a bit more, most notably reliable communication with signals (better than current cab signal systems). There are sub-PTC safety systems available, such as train stop devices (which can be rigged for overspeed protection). These are still expensive to install though and at this point most everyone wants to go directly to PTC, which provides much greater protection.