On May 12, 2015, an Amtrak train derailed in Philadelphia, PA, killing 8 and injuring 200 people. I saw a story on the incident where multiple NTSB spokespeople have said at this and other train derailments that Positive Train Control (PTC), had it been installed, could have prevented the derailments from happening, particularly in incidents where human error is the key factor, rather than technical failures or track obstructions. PTC uses GPS and transponder technology to avoid collisions and derailments by ensuring that trains operate within speed limits and do not cross improperly-switched tracks.

As the first link points out, the Rail Safety Improvement Act of 2008 (passed in the wake of the Chatsworth train collision) mandates that much of the nation's rail system have PTC installed by Dec. 31, 2015. However, the rail industry has been lobbying to extend the deadline, claiming that they will be unable to have PTC installed and operational on all required lines, and be forced to either cease operations, or operate illegally.


  1. What is required to implement Positive Train Control on existing rail lines in the United States? Does implementation require track improvements, engine/car improvements, or both?

  2. Are the systems that are currently used or in development inter-compatible? For instance, a commuter rail system near me uses existing BNSF and Union Pacific lines (among others) in addition to lines that it owns itself, and Amtrak operates on some of these lines as well. Do all trains operating on these lines need identical PTC systems, or can the commuter rail develop a different but compatible system than the freight rail?

  3. How long have complete rollouts of PTC taken, and what have they cost? (This is a fact-based way of answering the more debatable question 'Is 7 years a reasonable period of time to allow for PTC development and implementation?' and to examine what kind of budgetary strain the unfunded mandate places on smaller rail systems.)


1 Answer 1


Speaking personally (I work for a US Class I as a software developer, working with a system that will interface closely with the PTC implementation at my company and knowing developers who have worked on parts of that PTC implementation):

  1. The main physical hardware improvements required to implement AAR-type PTC on a rail system are fourfold:

    a. The existing wayside infrastructure must be brought up to a point where it can be reasonably interfaced to the PTC system -- relay interlocking, for instance, must be replaced with solid-state, microprocessor interlocking.

    b. The PTC-specific infrastructure must be fitted to the wayside. This includes switch position detection and signal system interfacing, as well as the telecommunications (radio towers and backhaul) infrastructure required for PTC data radios.

    c. PTC systems must be fitted to all locomotives operating over the network -- this includes the onboard computers, data radios, and displays.

    d. Finally, a back-office system must be put in place to pass authority, switch and signaling, train, and track bulletin information back and forth between the trains on the system, the wayside units, and other components (such as the dispatching system) as well as foreign railroads.

    Atop this, there is also the need to gather extremely precise and accurate data about the permanent way (where track is located) and equipment mounted to it (such as switches, signals), as well as designate locations along the track (stations, mileposts, etal) precisely. This is not easy -- while track geometry recording cars capture much of this data, they are only run periodically, and are vulnerable to operator error as well.

  2. The design of the current PTC systems is intended to be intercompatible; the Association of American Railroads has set communication standards for use in the PTC system to try to ensure this. However, we won't know what bugs will shake out until we test this in the field, of course.

  3. If you are asking about an individual PTC implementation, the first adopter of this technology was, ironically, Amtrak, who has the Advanced Civil Speed Enforcement System or ACSES in place on most of the Northeast Corridor in order to allow high-speed Acela operations there; this was deployed in 2000. ACSES uses much of the same technology as will be deployed in other PTC installations, and can be bridged to foreign railroad PTC systems; however, it is not quite identical in protocol to future PTC deployments, and is designed to overlay over existing coded cab signals instead of replacing them. However, the NEC lacks the volume of traffic, the amount of hardware, and the quantity of inter-railroad interchange that the freight lines of the US have, which makes the problem significantly simpler for Amtrak to handle.

  • $\begingroup$ Unfortunately, I do not have implementation-time or cost numbers for ACSES handy -- if another user can provide an answer with those, please do! $\endgroup$ Commented May 19, 2015 at 23:15

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