For our acronymious alphabet soup lovers, a synopsis of FRA info from the website (as of 2017, since then there may have been some changes -- I think a few have dropped out of the running):

Types of PTC Systems in the United States

ACSES (Advanced Civil Speed Enforcement System): A transponder-based system, in use on Amtrak's Northeast Corridor originally put into use on the Northeast Corridor by the specific requirements of an Order of Particular Applicability. (Type Approved and Certified by FRA.)

ACSES II: The latest version of the Advanced Civil Speed Enforcement System, and acts as a vital overlay to an Automatic Train Control (ATC) system comprised of a Cab Signaling System (CSS) and a Speed Control System (SCS). (Type Approved and Certified by FRA.)

CBTC (Communications-Based Train Control): A vital stand-alone Positive Train Control (PTC) system, as defined in 49 Code of Federal Regulations (CFR) Part 236, Subpart I, Section 236.1015(e)(3). CBTC replaces the existing traffic control method of operation by requesting an override of the wayside signal system to display a Flashing Green or Flashing Yellow (if the green aspect does not illuminate) signal aspect. (Type Approved and Certified by FRA.)

ETMS (Electronic Train Management System): A GPS- and communications-based system previously deployed by BNSF Railway, originally put into use by the specific requirements of 49 CFR Part 236, Subpart H. (Type Approved and Certified by FRA for restricted use.)

I-ETMS (formerly called Vital Electronic Train Management System): A GPS- and communications-based system ready for deployment. It is the system of choice for CSX Transportation, Norfolk Southern Railway, Union Pacific Railroad and BNSF Railway. Various passenger/commuter and other railroads are adopting it for compatibility and interoperability. (Type Approved by FRA.)

ITCS (Incremental Train Control System): A GPS- and communications-based system used by Amtrak on its Michigan line, authorized for passenger train speeds up to 110 mph, originally put into use by the specific requirements of an FRA-approved waiver. ITCS certification through Amtrak's request for expedited certification process is pending successful resolution of a few remaining issues prior to FRA approval for certification.

E-ATC (Enhanced Automatic Train Control): A system that uses an underlying automatic train control (ATC) system, in conjunction with other "enhanced" features or systems to achieve the core required functionalities of PTC. These systems are often integrated with underlying cab signal systems (CSS) and centralized traffic control (CTC) systems, in addition to other signal or train control system enhancements the railroad elects to make, to meet the full requirements of PTC.

I-ITCS (Interoperable Incremental Train Control System): A safety-critical, overlay system as defined in 49 CFR Part 236, Subpart I, Section 236.10 15(e), to be used in conjunction with the existing method of operation (Traffic Control System). I-ITCS interoperability is achieved by incorporating the same capabilities as the Interoperable Electronic Train Management System (I-ETMS). (Type Approved by FRA.)

SafeNet System: The Argenia Railway Technologies' Positive Train Control (PTC) system (SafeNet System) is a non-vital overlay PTC system, as defined in 49 CFR § 236.1015 (e)(1). The SafeNet System is designed to support the Nashville & Eastern Railroad (NERR) and its method of operation. The SafeNet System will be used in conjunction with the existing method of operation, centralized traffic control (CTC), which interfaces with existing signal systems, wayside devices, and office train dispatching systems Computer-Aided Dispatch (CAD) via multiple communications links. (Type Approved by FRA.)

Sentinel System: An overlay Positive Train Control (PTC) system, as defined in Title 49 Code of Federal Regulations (CFR) Part 236, Subpart I, Section 236.l 015( e )(1 )(ii). This CBTC system is overlaid on the existing method of operation and provides enforcement of movement authority limits, maximum authorized speeds, permanent and temporary speed restrictions, and incursion into roadway work zones by a positive stop. (Type Approved and Certified by FRA.)