Slowly everything is getting back to normal. I'm posting these with a historical intent. Links where possible, and let me know if your local is missing.
NOTE: MOST AGENCIES ARE ASKING FOR ESSENTIAL TRAVEL ONLY -- MEDICAL AND SUPPLY RUNS, JOB TRANSIT FOR THOSE WHO PROVIDE THOSE SERVICES. STAY HOME! EXPECT DELAYS AS THEY RUN CRITICALLY LOW ON STAFF.
Many agencies are asking you wear a mask, board from the back, and are putting limits on passenger boarding. Mask requirements are marked with "!!!" States that require a mask: MD, MI, NJ
If the agency has a pass app, please pay for the ride through it -- it helps the agency!
(I have also taken the time to clean up and organize the list a bit)

Country-wide

AMTRAK: Covid-19 info
Note: All overnight trains (except Auto Train) will switch to Flexible Dining menus. Some Cafe service is suspended. Limited access to NY Penn.
Reduced: NEC, Hartford Line, Thruway Buses, Capitol Corridor, Empire Service, Ethan Allen Express, Illini/Saluki, Illinos ZephyCarl Sanburg, Lincoln Service, Missouri River Runner, Pacific Surfliner, Piedmont, San Joaquins, Valley Flyer, Vermonter (no service north of New Haven M-Sat, no service Sun), Wolverine
Split Service: California Zephyr (no service Reno to Denver), Palmetto (NY to DC only April 3-5)
Terminated before Canada, Reduced service: Maple Leaf (terminates Niagra Falls), Adirondack (terminates Albany, NY), Cascades (terminates Seattle).
Canceled: Acela, Keystone, Pennsylvanian, Winter Park Express, Pere Marquette, Carolinian, Downeaster
Replaced w/bus (Thruway): Hiawatha (4/24-5/25)
VIA RAIL (CANADA): Covid-19 advisory
Meal service is reduced to snacks and drinks. Some sleeper service is canceled. VIA Rail will refuse passengers who are sick w/o doctors note.
Canceled until May 1st: The Ocean (Montreal to Halifax), Toronto-Niagara Falls (until further notice)
Canceled until June 1st: JaspePrince George/Prince Rupert, The Canadian (Toronto to Vancouver)
Limited service: Montreal-Jonquiere/Senneterre (Friday/Sunday departures), Sudbury-White River (Weekend only), Quebec City/Winsor corridor (Kingston-Torronto-London is canceled, most trains canceled)

State/Provence and City

Shut down/demand ride only

Michigan:

• KMetro (Kalamazoo)
• STARS (Saginaw)
• Bay Metro (Bay City)
• Lake Erie Transit

Reduced Service - Canada - Improving

• MiWay Mississauga, Ontario: 8 buses resuming regular schedule, 1 express improving, 8 reduced, 19 canceled

Reduced Service - Canada

• Toronto Metrolinx GO Transit: Trains -- very limited service. West Harbor, St. Catharines, Niagra Falls stations closed. Buses -- Futher reduced. UP service -- 30 minutes
• Toronto TTC: 1 Bus and almost all Express buses canceled. Streetcars -- 503 truncated, 508 canceled.
• York Regional Transit (Toronto): 12 buses suspended. Revised schedules elsewhere.
• Calgary Transit: CTrain 7-16 minute headways depending on time of day; buses adjusted for limited service.
• Saskatoon Transit No University/High School runs. M-F 15 High-freq/30 min rest frequencies before 7pm, 30/60 after. Sat 30/60 (regular 30min 11a-7p). Sun 30/60.
• Regina, Saskatchewan: Sat service M-F, Sun service on Sat.
• Quebec RTC: (In French, no English available) Reduced service. No school, late night service. Handicap service only via paratransit.
• Montreal Transport Society: Bus -- Reduced schedules, some lines to closed schools, parks, casinos closed. Rail -- Reduced frequency of Orange/Green lines.
• Translink Vancouver: SeaBus 30-min frequency, Reduced frequencies on low-ridership lines, SkyTrain reduced frequencies, West Coast Express 2 trains canceled. 24 bus routes suspended(41 in mid-May)
• Edmonton Transit: Saturday-plus-rush-extras, Light Rail ends at 10pm, OWL service midnight. Late-nite on-demand medic-only service started. 40' buses being used, some routes detoured to accomidate. 747 bus line for job travel only (no travelers/shoppers).
• Medicine Hat Transit (Alberta): M-F on Saturday schedule, Sa-Su on Sunday schedule.

Reduced Service - USA - Improving

• San Jose VTA: !!! Light Rail back operational 6a-6p M-F. No school trip service. Reduced bus service, some suspended, ends 9pm.
• Columbus Ohio COTA: !!! 17 lines, plus AirConnect, CBUS, and Night Owl, canceled. Early morning service on 6 lines resuming. Modded service on other lines.

Michigan

• CATA (Lansing): !!! Saturday service M-F. No Shopping Bus.
• The Rapid (Grand Rapids): !!! Service hours 7a-7p. No DASH/May Mobility service. 5 routes 30 min headways M-F, hourly Sa/Su. 8 routes hourly everyday. Silver Line 30 min headways everyday. Route 50 on 50 min headways, 85 on 25 min headways.

Reduced Service - USA

East Coast

• Boston MBTA: Commuter Rail reduced schedule. No ferry service. Saturday schedule everywhere else with some bus exceptions. Saturday-plus-extra on Blue and Green E lines.
• New York City MTA/Metro-North/LIRR: Reduced service. Subway closed 1a-5a. Penn Station limited access 1a-5a. No B, C, W, Z, 42nd street shuttle, or express 4/5/6/7. J is Local. No extra L service. Some lines run local in areas. 5 is cut down. Buses down to 75% service. Staten Island Railway hourly service. LIRR 30/60 min-plus-rush schedule, cross-honoring MetroCards inside city zone. Metro North East-of-Hudson on hourly-with-rush-extras, Waterbury branch bus substituted (Wassaic branch suspended on weekends), West-of-Hudson see NJ Transit.
• NJ Transit: !!! Commuter rail - President's Day schedules until further notice. Gladstone branch on weekday service only. 7600 series trains canceled. Atlantic City Rail Line regular service. Bus -- Saturday service w/exceptions (and cancelations). Light Rail -- HBLR, Newark lines Saturday schedule. River Line Sunday schedule.
• SEPTA Philadelphia, PA: Regional Rail -- Limited service on Airport, Fox Chase, Lansdale (no extensions), Media/Elwyn, Paoli (no service to Malvern), and Trenton; all others canceled. Subway -- Some stations closed. Saturday service between 4:30am and 1am. No overnight OWL. Trolley -- Headway based Saturday schedule. 102, 34 lines canceled, 101 bus subsitution. Everything else -- Headway-based Saturday service.
• PATCO Philadelpha, PA & New Jersey: 4 stations closed. Limited service. Headways M-F: 12a-4a hourly, 4a-5a 30 min, 6am-midnight 20 minute. Sa: 12a-5a hourly, 5a-12a 30 minute. Su: Hourly all day.
• Pittsburgh Port Authority: !!! Reduced bus and light rail service.
• Maryland MTA: !!! MARC Commuter rail -- Reduced-from-"R" service. Light Rail, Metro subway -- M-F runs Saturday schedule. 1 station limited access. Commuter Bus -- 1 route suspended, 13 S-service, remaining limited service. Bus -- No Express service. 2 Local buses canceled. Saturday-plus-rush-extra service.
• Washington DC WMATA: Bus -- Sunday-plus-extras schedule, 27 line "lifeline" weekend service w/30 min headways. No service after 11pm. Subway -- 19 stations closed. 9 limited access. 15-20 minute headways, 5a-9p M-F, 20-30 min headways 8a-9p Sa/Su.
• Altanta, GA MARTA: Rail -- 20 min headways on Sunday service. Red/Gold to airport, Green terminates at King Memorial. No service at Arena station. Streetcar -- Sunday schedule. Bus -- 41 bus lines operating, remaining suspended.

Mid-country

• Cincinnati Go*Metro: Streetcar service canceled where bus service exists. Express buses canceled. Saturday service everywhere else.
• Chicago RTA (Metra/CTA/Pace): Metra Rail -- Reduced service except Heritage corridor. Adjusted PACE schedules to accommodate.
• Minneapolis/St. Paul Metro Transit: Overnight service suspended 11pm-4:30am except airport shuttle. Reduced bus service, based on Saturday. Maintenance on some lines April 20-27.
• St. Louis Metro: Weekend-with-extras service on weekdays. 9 buses, Missouri Express routes suspended.
• New Orleans RTA: Saturday schedule Mon-Fri. 10 bus lines suspended. Streetcar 32-36 min frequency, 2 lines shut down. No service 10p to 4a.
• DFW TRE: Changed schedules.
• Dallas DART: Light Rail -- 20 min headways. Bus -- 2 routes canceled.
• Kansas City RideKC: KCMO/KCK service on Saturday schedule ending 9pm. Johnson County service 60 min, reduced express. Some buses canceled.
• Albuquerque ABQ Ride Transit: Saturday service. ART service suspended entirely.
• Rio Metro/Rail Runner New Mexico: Rail Runner is suspended until May 15th. 3 buses suspended. Some buses limited.
• Phoenix, AZ Valley Metro: Service hours 5a-11p M-F, Sa/Su 6a-11p. Orbit, Express Bus, RAPID bus routes reduced. No POGO service. Scottsdale trolley service suspended. Light rail limited.
• Salt Lake City/UTA: Commuter rail "FrontRunner" smaller trains, hourly service. TRAX service every 30 minutes. Bus service curtailed, some commuter bus routes canceled.
• Cascades East Transit (Bend, Oregon): !!! Saturday service, demand ride available. In Bend: no service Saturday, no demand ride Saturday/Sunday.

West Coast

• Seattle Sound Transit: Link Light Rail -- 20 min headways. Sounder service reduced. Most buses reduced. 10 buses suspended.
• Portland TriMet: MAX trains 15 minute service, weekends Sunday service. WES 45 min service. Reduced bus service.
• San Francisco Muni MTA: 17 buses running, additional 9 limited service. No rail service -- replaced by buses. (Improvements expected April 25th)
• San Francisco BART: !!! System closes early at 9pm. 30 min service. Limited access at many stations. Early Bird Express -- limited trips, 2 lines suspended (more April 27).
• CalTrain (California): Local service between San Francisco and San Jose 30-60 minute frequency, 4 trains to Gilroy peak commute. Limited/Baby Bullet service suspended.
• Los Angeles MTA: Rail service ends at Midnight. M-F 12 min headways 6a-6p, 20 min remaining times on A,B,D,E,L lines. C line 12 min 4a-9a, 15 min 9a-3p, 12min 3p-6p, 20 min 6p-midnight. G/J bus subsituted. B/D stations limited access. Bus service: Most lines Sunday schedule. 23 lines hourly service. 15 Rapid lines on modified weekday schedule. 14 lines suspended.

Michigan

• SMART (Detroit suburbs) is running with vastly reduced schedules and 13 suspended commuter lines.
• DDOT (City of Detroit) is running Saturday schedules Mon-Sat with free fares and rear boarding. 8 bus lines suspended.
• TheRide (Ann ArboYpsilanti): Sunday reduced schedules with tweaks. 12 buses shut down.
• BATA (Traverse City): !!! Village Loop shut down. Village Link demand ride service replaces it. Reduced schedules other buses.

Hat-Tips

u/alitales u/NixillUmbreon u/Steaks_on_a_Plane u/voxadam u/cincy_transit_guy u/mohaas06 u/taxorenxuan u/earlyenrichment u/SojuCondo u/TTC_Tokens u/TC1827 u/InfiNorth u/Bombpants u/sebileis u/Pwheatstraw2000 u/Vectrex452