Reply to "Required use of Mars light?"

Add these original owners:
MP (passenger; also a small number of SD40's bought for pool service with D&RGW)
T&P (passenger, freight, hood units)
Union Pacific until 1955 (passenger, freight, GP7, SD7; not on E9's built 1955 and later, or on GP9 and later hood units)
Akron Canton & Youngstown
Denver & Rio Grande Western (passenger, freight)
Burlington Route (passenger, freight F3 & F7, hood units up to BN merger; added to steam 4-6-4, 4-8-4, 2-10-4, FT's, F2's, and EMC shovel-nose Zephyr power cars)
C&NW/Omaha Road (passenger, freight, early BLW/F-M road switchers, later all removed; also some steam passenger engines)
Southern (passenger DL-109's, E-units and PA3's, later removed from E-units)
Northern Pacific (passenger, freight)
Nashville Chattanooga & St.Louis (passenger, freight, hood units)
Louisville & Nashville (passenger, freight, hood units including RS3's)
Monon (passenger, freight, second generation hood units)
M-K-T (passenger, including FP7; also hood units equipped for passenger service)
Frisco (passenger E-units; later added to freight cab units)
Pennsylvania (2 Baldwin passenger cab units, and 2[?] E8's)
Atlantic Coast Line (passenger, freight, hood units)
Seaboard Air Line (passenger)
Canadian Pacific (RDC's; also, later, FP7/FP9, on roof, angled upward)
Canadian National (RDC's)
Great Northern (passenger)
Milwaukee Road (passenger, freight F7's & F9's [but not FT's], some steam engines)
Rock Island (DL-109's, E-units, F2's, AB6's; later added to EMC TA's and 1 or more [but not all] FP7's)
Florida East Coast (passenger, freight, hood units)
Western Pacific (passenger, RDC's, second-generation hood units)
Spokane, Portland & Seattle (passenger E7 & F3's)
Wabash (passenger, freight, first-generation hood units)
Gulf Mobile & Ohio (PA1's, Baldwin passenger units; former Alton E7's not equipped)
Soo Line/Wisconsin Central (passenger, freight, GP7, GP9)
Chesapeake & Ohio including Pere Marquette (passenger, some GP7's (removed from GP7's)
Delaware & Hudson (PA's, second-generation hood units)
Erie-Lackawanna (3 units involved in trade with D&H? - needs verification)
Green Bay & Western (FA1's)
Tennessee Central (FA1's)
Amtrak (SDP40F's, P30CH's, GE electrics)

Chicago Great Western (passenger, freight, GP7's, second-generation hood units)

Trona Railway

Nevada Northern (1 SD7, later removed)

Most northeast carriers declined to equip their engines with Mars lights.  It's just a general observation, but most of the carriers which used them ran in parts of the country with wide-open landscape and/or many road crossings with only passive protection (crossbuck signs) as opposed to the many grade-separated crossings or crossing gates of the northeast.  D&RGW notably used them also for observation, as it has miles of track in narrow, deep canyons.