Here's a list of streetcar operators in the USA and the vehicles they use today!
SunLink in Tucson, AZ using United Streetcar 200s
Metro Streetcar in Little Rock, AR using Birney replicas by Gomaco Trolley Company.
San Francisco Cable Car streetcars in San Francisco, CA using cable cars manufactured in 1878
San Francisco Heritage Streetcars in San Francisco, CA using PCC streetcars manufactured in 1936 by either St. Louis Car or Pullman.
Fort Collins Municipal Railway in Fort Collins, CO using a Birney manufactured by American Car Company in 1921.
DC Streetcar in Washington DC, using United Streetcars models based upon Skoda 10T
TECO Line Streetcar in Tampa, FL using Birney replicas by Gomaco Trolley Company.
Atlanta Streetcar in Atlanta, GA using Siemens S70.
New Orleans streetcars in New Orleans, LA using Perley Thomas manufactured in local shops in 1921, and new replicas using Brookville Equipment trucks.
MBTA's Ashmont–Mattapan High-Speed Line in Boston, MA using PCC streetcars manufactured by Pullman in 1929.
Q-Line in Detroit, MI using Liberty streetcars manufactured by Brookville Equipment.
KC Streetcar in Kansas City, MO using CAF Urbos 3.
Loop Trolley in Saint Louis, MO using Brill-replicas by Gomaco Trolley Company.
CityLynx Gold Line in Charolette, NC using Siemens S70.
Bell Connector in Cincinnati, OH using CAF Urbos 3.
El Reno Heritage Express in El Reno, OK using J.G. Brill Strafford Car manufactured in 1924.
OKC Streetcar in Oklahoma City, OK using Liberty streetcars manufactured by Brookville Equipment.
Portland Streetcar in Portland, OR using various manufacturers including Škoda, Inekon, and United Streetcar.
SEPTA subway–surface trolley lines in Philadelphia, PA using Kawasaki Type K
MATA Trolley in Memphis, TN using refurbished Porto, Portugal and Melbourne, Australia W2-class cars.
El Paso Streetcar in El Paso, TX using refurbished PCC streetcars by Brookville Equipment.
Dallas Streetcar in Dallas, TX using Liberty streetcars manufactured by Brookville Equipment.
M-Line Trolley in Dallas, TX using various heritage manufactured streetcars including J. G. Brill Company, St. Louis Car Company, Melbourne W2, American Car Company, and Brussels-built Westinghouse PCC.
S Line in Salt Lake City, UT using Siemens S70.
Seattle Streetcar in Seattle, WA using Inekon Trio-12, and CAF Urbos.
T Link in Tacoma, WA using Škoda 10 T, and Brookville Liberty.
Kenosha Electric Railway in Kenosha, WI using refurbished PCC streetcars manufactured by St. Louis Car.
The Hop in Milwaukee, WI using Liberty streetcars manufactured by Brookville Equipment.
As anyone should easily see, there is a wide variety of ages, replicas, new, trams from different countries, and different manufacturers for any national standard to arise. Back during FDR's Presidential term, PCC was pushed as a national standard. It never became one. None ever will, imho.
Additionally, almost all are funded and operated with local money, with some Federal grants to get new lines running. Few Federal grants are provided to operators, private or public, for operations. Some are ran my volunteers, some by larger transit agencies. As long as most of the funding to subsidize them is local, local requirements will rule the roost.