Here's the link to the Regulation, https://www.ecfr.gov/cgi-bin/text-idx?S ... 0&rgn=div8
, "e-CFR data is current as of November 14, 2018". That "quote from a 2015 regulation" is what will kick in shortly. It has only been postponed from Dec. 31, 2015 by amendments.
This link to Appendix I of Title 49 of the Code of Federal Regulations, Part 236 gives the details of Positive Train Control. https://www.ecfr.gov/cgi-bin/text-idx?S ... i&rgn=div6
Where does it say that Cab Signals must be part of Positive Train Control?
In that link are the new dates for requiring Positive Train Control:
236.1005 Requirements for Positive Train Control systems.
(6) New rail passenger service. No new intercity or commuter rail passenger service shall commence after December 31, 2020, until a PTC system certified under this subpart has been installed and made operative.
(7) Implementation deadlines. (i) Each railroad must complete full implementation of its PTC system by December 31, 2018.
(ii) A railroad is excepted from paragraph (b)(7)(i) of this section and must complete full implementation of its PTC system by December 31, 2020, or the date specified in its approved alternative schedule and sequence, whichever is earlier, only if the railroad:
(A) Installs all PTC hardware and acquires all spectrum necessary to implement its PTC system by December 31, 2018;
(B) Submits an alternative schedule and sequence providing for implementation of positive train control system as soon as practicable, but not later than December 31, 2020;
(C) Notifies the Associate Administrator in writing that it is prepared for review of its alternative schedule and sequence under 49 U.S.C. 20157(a)(3)(B); and
(D) Receives FRA approval of its alternative schedule and sequence.
(iii) If a railroad meets the criteria in paragraph (b)(7)(ii) of this section, the railroad must adhere to its approved alternative schedule and sequence and any of its subsequently approved amendments or required modifications.
If S.E.P.T.A. and Amtrak want to have stricter rules, they are allowed.