1940s to 1950s era?
Alco, Baldwin, Fairbanks Morse, and EMD were the big players back then. This eventually distilled down to General Electric and EMD that we have today.
Alco had FA1 and FB1's for freights. These were the A and B units. The A units have the cab while the B units are just extra generators without a cab. Their passenger equivalent was their PA1 and PB2.
They had the S and RS series shunting locomotives. The RS series could also be used in passenger service and some were outfitted with steam boilers and generators for heating, power and air-con for the passenger cars. These were mostly used for commuter service. In addition to switching, RS units could be used on the road as well such as the RS3.
Fairbanks Morse mostly made large shunting locomotives that were also used in commuter service. The HH series were their big shunting and commuter locomotives. They also made a large passenger locomotive and were involved in a TALGO type train. The Boston and Maine and New Haven had these.
Baldwin had switchers and big road locomotives. They are more famous for their shunting locomotives.
EMD had a series of road diesels. On the road, they had FTs, and later E-series, and F-series. The F-series lasted until the 1980s and are still found today in some areas. Australia has their own version of them. In the 50s, EMD developed the Geep with the earliest being the GP7 and shortly afterwards the GP9. These are still found in operation on many short lines and in switching duties today on the bigger railroads. EMD's switchers (shunters) were SW-series. There are many older models still in operation such as the SW9s and SW1200s.
GE made some famous center-cab locomotives, some of which are still in use today. Their 44-tonners, 60-tonners, and 70-tonners are powerful shunters that have a tight turning radius and found a home switching mills, warehouses, and docs.
Lest we not forget, there is the Budd Rail Diesel Car, aka the RDC. These Budd RDCs were famous and some are still in use today after arriving in the 1940s. They are found worldwide including Australia, Saudi Arabia, and of course the US and Canada.
There were some smaller companies that eventually disappeared. Here you'll find Plymouth and Lima. Lima didn't last too long and was merged into Baldwin. Lima, like Plymouth, made shunting locomotives. Plymouth lasted until the early 70s making small shunting locomotives.