The ones with the most built, yes. Many of those are practically gone.
The major diesel manufacturers:
EMD (Electro-Motive Division of General Motors, now Electro-Motive Diesel) and its Canadian associate, General Motors Diesel: Builder of the first road freight diesel to see large-scale production (the FT, during World War II), EMD quickly became the dominant builder. In the late 50s, they had 80% of the market. This was sustained for a long time by the reliability and long service lives of EMD products compared to all competitors. Now, GE sells more new engines, but EMD's historical total production is still much greater.
GE (General Electric): In the very early days, collaborated with Alco on boxcab diesels. Continued to supply Alco with components while building industrial switchers and export units. Struck out into the domestic mainline market with the prototype XP24 in 1959. The first production units of this model, now termed the U25B, were sold in 1961. GE increased in market share until, in 1983, it finally sold more engines than EMD. Since then, GE has always been on top.
Alco (American Locomotive Company), MLW (Montreal Locomotive Works) and Bombardier: Alco was consistently #2 in the transition era. Since they were an established steam builder and EMD wasn't, War Production Board restrictions and self-competition got them off to a slow start and prevented them from ever being #1. GE took the #2 position in the 60s, and there wasn't much room for a #3. Alco shut down in 1969. For a long time MLW had been builidng Alco designs in Canada, and now built their own developments from Alco designs. MLW built its last engine in 1977, but Bombardier continued development and production until 1984.
Fairbanks-Morse and CLC (Canadian Locomotive Company): The other major builders sold boxcabs, switchers, streamliners and/or passenger engines before WWII. FM, a builder of diesel engines for marine and stationary power generation applications, broke into locomotive building after the war. Unlike all other diesel builders, they used opposed-piston engines (half the pistons go up, half down). These engines served well in other applications, and they allowed FM to build more powerful locomotives for their size than their competitors could at the time. However, they were difficult to maintain, and the electrical systems weren't very good, either. FM stopped building locomotives in 1963. Throughout its short career, CLC licensed FM designs in Canada.
Baldwin and BLH (Baldwin-Lima-Hamilton): Baldwin was a major steam builder. They built diesels in the early days, and sold decent quantities of their switchers. As they built mainline engines in the postwar era, some things became clear. Baldwin engines were powerful, but achieved this through brute-force engineering. Their prime movers were notoriously unreliable, and their electrical systems were nothing to write home about. They stopped building in 1956.
Lima and Lima-Hamilton: A significant steam builder, Lima made very few diesels. None were sold west of the Mississippi, in fact. They were absorbed by Baldwin in 1951, forming BLH, which produced only Baldwin designs.
Morrison-Knudsen, now Motive Power Industries: A general heavy engineering company, M-K got into the locomotive rebuilding business in the early 70s. Most of their work was rebuilds on EMD locomotives. In 1991, M-K built its first engines from the ground up: F40PH-2Cs, virtual clones of the EMD model of the same name. In 1997, it changed its name to MPI.
This lists builders that made engines for railroads, and does not count a number of builders that made small industrial switchers primarily or exclusively.