Advanced Civil Speed Enforcement System
Advanced Civil Speed Enforcement System (ACSES) is a positive train control cab signaling system developed by PHW and Alstom. The system is designed to prevent train-to-train collisions, protect against overspeed and protect work crews with temporary speed restrictions. The information about permanent and temporary speed restrictions is transmitted to the train by transponders lying in the track, coded track circuits and digital radio. It is installed on parts of Amtrak's Northeast Corridor between Washington and Boston.
General system design 
ACSES enforce speed restrictions upon trains to improve safety by mitigating against human error. It applies train brake systems depending on the information about speed restrictions transmitted to the train.
There are two kinds of speed restrictions, which are enforced by the system:
- Permanent Speed Restrictions are applied to trains to ensure that the maximum safe speed for track conditions and other environmental factors are not exceeded. These speed restrictions are also called civil speed restrictions.
- Temporary speed restrictions are applied to trains in the event of maintenance crews working on wayside equipment along the tracks to improve worker safety.
In the event a train's crew exceed a speed restriction the train's brake systems are applied to reduce the speed to comply with the restrictions. In the event that braking is ordered by the onboard computer system, event recording is performed that records the fact that braking was required so that railroad managers can determine whether additional training is needed for their onboard locomotive crews.
Speed restrictions required by the signal system are provided by the legacy pulse code cab signals system, which has been in service on various railroads since the 1930s. The cab signal codes are fed into the ACSES cab display unit, which then enforces the more restrictive of the two speeds. The on board ACSES unit is backwards compatible and can function where only the cab signaling is present without the ACSES overlay.
In addition to ensuring maximum safe speed, the system also ensures maximum return on investment by safely authorizing a locomotive’s maximum speed. Locomotives need not come to a halt while onboard crew communicate with their dispatch controllers via radio to determine whether speed restrictions are in effect or whether maintenance crew are in possible harm’s way. Locomotives know in advance of any such restrictions, allowing them to proceed at best safe speed without interruptions.
ACSES meets definition of Positive Train Control 
The American Railway Engineering and Maintenance-of-Way Association (AREMA) organization describes what it means by Positive Train Control  and among the most salient aspects are:
- Train separation (collision avoidance)
- Civil speed restrictions enforcement
- Temporary speed restrictions
- Rail worker wayside safety
The ACSES system meets these core requirements.
On-board equipment 
The on-board equipment consists of an on-board computer, an antenna subsystem, and a radio subsystem. In the cab, the driver has a consolidated display which displays the train's target speed and other useful operating information.
Messages conveyed to and from locomotive and ground-based systems are made up of ATCS-encoded (Advanced Train Control Systems) message frames.
Field equipment 
The system begins with passive transponders buried under the tracks which are electrically powered by an electromagnetic field when a locomotive passes over them. The transponders digitally convey their identification information and other relevant bits of information via wires to a trackside Safetran Base Station Controller (BCM), which uses a Safetran radio to communicate with the locomotive’s onboard computers, allowing the locomotives to know precisely when they have reached a particular waypoint along the track.
The onboard systems examine its database of speed restrictions, constantly ensuring that the maximum safe speed for any given location along the track is not exceeded.
As the locomotive proceeds down the track, the onboard systems communicate via radio to the trackside BCMs in the region, requesting any temporary speed restrictions for the next three or more regions of the track, ensuring that the locomotive’s database is always kept current with any possible temporary restrictions issued by the dispatch and control office equipment.
Also in the field are Safetran Wayside Communications Managers (WCM) (or packet switches), which communicate with BCMs in the region via radio which also communicate with the office dispatch and control systems via TCP/IP.
This design provides locomotives with positive information about where they are and what their maximum safe speeds are, and it also keeps the office informed about where the locomotives are located.
In addition to speed restrictions, information about the tracks ahead can be acquired by the ground network which informs locomotives of the state and status of the tracks which the locomotive are approaching. Such information about the status of the track ahead—occupancy, switch position, signal light states, and a host of other vital inputs—is accumulated by wayside encoders such as the Safetran VIU-ACSES (shown in the photograph to the right) and then the information is sent to the BCMs for transmission to locomotives.
Office equipment 
In the office where dispatch and control is performed, a system provides a visual indication of the current status of communications with all locomotives as well as a close approximation of where each locomotive is currently located along the track.
In the event that maintenance is needed along any section of the track, before a work crew is dispatched or before a work crew is granted authority to proceed, a temporary speed restriction (TSR) is created in the office computer systems. After a series of verifications and procedures, the TSR is presented to the ACSES office system.
When a locomotive issues a query for TSRs for a given region, the WCM conveys the request for information to the office system via TCP/IP and the response is conveyed back to the locomotive which updates its local database with any restrictions.
There are a number of redundant components in the overall ACSES system such that a failure of a subsystem will swap over to another automatically. The loss of a WCM, for example, due to a power outage or lightning strike results in a standby WCM taking over the communications duties between BCMs and the office systems.
In the event of a loss of all redundant standby systems (such as might occur in a catastrophic environmental condition as might be expected with a hurricane or forest fire) the entire system immediately drops to its failsafe states.
Because a locomotive's radio is capable of being heard by a number of BCMs, the WCM examines the indication RF signal strength of each BCM that heard the locomotive to determine what the strongest talk path back to the locomotive is. The WCM maintains a record of three possible talk paths to the locomotive such that the strongest path is always selected if the office needs to communicate back to the locomotive.
As a locomotive moves from region to region, the radio signal strengths recorded by BCMs which get conveyed to the WCMs change. BCMs which fall out of range of locomotives are removed from talk path routes within the WCM in favor of the BCMs which are coming into range.
In this way the WCM is constantly aware of where each locomotive is located and which talk path is best used to communicate with the locomotive. Such information is also conveyed to the office so that office systems may make use of it.
Another aspect of redundancy is the system design which looks forward along the track, acquiring TSRs for the future in the event a temporary communications failure occurs. Since each locomotive has TSRs for at least three future segments of the rail line, in the event there is a segment of the track which for some reason has lost radio communication to the office, the locomotive has TSR information for the "dark" segment already before it proceeds into the dark segment.
Finally, the cab signals are considered a completely independent system that transmits a continuous stream of codes through the rails instead of via wireless transmission. Any fault in the ACSES overlay will not affect the cab signal system.
Failsafe operation 
In the event that a locomotive is unable to automatically retrieve temporary speed restriction information from its office control system despite repeated retries and standby backup systems, the failsafe fallback policies established for the railroad are adopted and traditionally the locomotives reduce speed and require a human operator to communicate with a dispatch center for authority to proceed.
With Positive Train Control in effect, a series of authorization steps are needed to convince the onboard computer systems to allow the locomotive to proceed at maximum safe speed after the onboard crew acquire such authorization from the dispatch and control center—usually via radio.
See also 
- "Advanced Civil Speed Enforcement System (ACSES)". ALSTOM Signaling. 2003. Retrieved 2007-11-17.
- "Amtrak, Alstom Will Test Acses At Pueblo - advanced civil speed enforcement system". Railway Age. 1998-08-01.
- http://www.arema.org/eseries/scriptcontent/custom/e_arema/comm/c37/05-01-08/AREMA_MP_23-2-1_New_2008_Mar_F.doc[dead link]