Question: Who governs the size classification of Caldwell County Railroad?

Context paragraphs:
Lytton Springs, Texas: Lytton Springs is an unincorporated community in northeastern Caldwell County, Texas, United States.  Located along Farm-to-Market Road 1854, it lies approximately 10 mi northeast of Lockhart, the county seat of Caldwell County.

Crawford County Railroad: The Crawford County Railroad ran from Girard to Walnut in Crawford County, Kansas, USA.  It was established on February 6, 1884, from the failed Nebraska, Topeka, Iola and Memphis Railroad.  The Crawford County Railroad lasted for nine days before being acquired by the Kansas Southern Railroad.

Lockhart, Texas: Lockhart is a city in Caldwell County, Texas, United States.  It is the county seat of Caldwell County.  According to the 2010 census the population of Lockhart was 12,698.  Lockhart and Caldwell County are within the Austin-Round Rock metropolitan area.

Butte County Railroad: The Butte County Railroad was a 31.5 mi class II railroad that ran from a connection with the Southern Pacific Railroad at Chico, California to the Diamond Match Company lumber mill at Stirling City.  The railroad operated from 1903-1915 and then became the Southern Pacific's Stirling City Branch.  From 1915 until abandonment in the 1970s the line was operated as the Southern Pacific's Stirling City Branch.  The Chico and Northern Railroad was a non-operating subsidiary holding company of the Southern Pacific Railroad that was created to acquire a 32.31 mile line from Chico - Stirling City from the Butte County Railroad.  Upon acquiring the line, Chico & Northern immediately leased the line back to the Butte County Railroad.  The Chico & Northern was dissolved into Southern Pacific in 1912 and never operated any of the line.

San Marcos Regional Airport: San Marcos Regional Airport (ICAO: KHYI, FAA LID: HYI) is a public use airport located in Caldwell County, Texas, United States.  It is four nautical miles (7 km) east of the central business district of San Marcos, a city that is mostly in Hays County.  The airport is owned by the City of San Marcos and operated by Texas Aviation Partners.  It is located east of the border of Caldwell County and Hays County.  Before it was operated as a civilian airport it was known as Gary Air Force Base.

Lenoir, North Carolina: Lenoir is a city in Caldwell County, North Carolina, United States.  The population was 18,228 at the 2010 census.  It is the county seat of Caldwell County.  Lenoir is located in the Blue Ridge foothills.  To the northeast are the Brushy Mountains, a spur of the Blue Ridge Mountains.  Hibriten Mountain, located just east of the city limits, marks the western end of the Brushy Mountains range.

St. Johnsbury and Lamoille County Railroad: The St. Johnsbury and Lamoille County Railroad (StJ&LC) was constructed in the 1870s as the Vermont Division of the Portland and Ogdensburg Railway to connect the Great Lakes with the seaport of Portland, Maine.  The westerly connection with the Great Lakes was never made.  The eastern end of the Vermont Division was leased to the Maine Central Railroad in 1912, and the remainder of the line became a subsidiary of the Boston and Maine Railroad.  The Boston & Maine operated their segment as the St. Johnsbury and Lake Champlain Railroad after 1925.  This segment was reorganized as the St. Johnsbury and Lamoille County Railroad in 1948.

Lenoir Downtown Historic District: Lenoir Downtown Historic District is a national historic district located at Lenoir, Caldwell County, North Carolina.  The district includes 41 contributing buildings and 2 contributing objects in the central business district of Lenoir.  It includes commercial, governmental, and institutional buildings in a variety of popular architectural styles including Art Deco, Art Moderne, Classical Revival and Tudor Revival.  Notable contributing resources include the Center Theater (1941), O. P. Lutz Furniture Company and Lutz Hosiery Mill (1939), Dayvault’s Drug Store (1937), Caldwell County Agricultural Building (1937), Courtney Warehouse (c. 1888), Masonic Hall (1901, 1959), Miller Building (c. 1900, c. 1920s), Confederate Monument (1910), Belk’s Department Store (1928), Lenoir Building (1907), J. C. Penney Department Store (1941, c. 1980s), Fidelity Building (1928), and U. S. Post Office (1931).  Located in the district is the separately listed Caldwell County Courthouse.

Railroad classes: In the United States, railroads are designated as Class I, II, or III, according to size criteria first established by the Interstate Commerce Commission (ICC) in 1911, and now governed by the Surface Transportation Board.

Caldwell County Railroad: The Caldwell County Railroad (reporting mark CWCY) is a Class III shortline railroad operating over 17 mi between Hickory and Lenoir, North Carolina.  The CWCY is operated by Southeast Shortlines, Inc., which also operates the Thermal Belt Railway.

Supporting paragraphs: Caldwell County Railroad Railroad classes
Answer: Surface Transportation Board