I find many rail cars rather repetitive with little difference other than color or railroad name. Most tankers, gondolas, boxcars, flatcars and passenger cars are mostly similar. Lionel and others have been good at producing fictional variations to make them more interesting for us model railroad fans like the giraffe and aquarium cars. I think locomotives, tenders and cabooses have more individuality. I enjoy the variations of this type of railroad equipment.
Tenders are some of my favorite type of cars. The November 2001 CTT has a good article on Postwar Tenders to show the different tenders and charts to show which tender usually went with which engines. Usually the larger engines look better with larger tenders and would need larger tenders as they use more fuel.
Lionel made a great number of coal tenders of the New York Central type from 1946 to 1952 and were used on 671, 2020, 2025, 2026, 2035 locos for examples. They were 7 3/8 inches long. This tender is tied for my “most number of tenders” in my collection. 
Many locos had large (8 1/8 inch long) streamlined tenders made from Pennsylvania streamlined style tenders and some had water scoops on the bottom and 6 wheel trucks like the one below. They were used from 1950 to 1968 for larger locos like 2046, 773, 2056, 671, 2065 and many more.
Small (7 1/8 inches long) streamlined tenders were made from 1954 to 1966 for a great many middle sized locos like 1062, 242, 2018, 2037 and many more. This is the other tender tied for my “most number of tenders” in my collection. I had so many I painted several to match painted Lionel 242 plastic bodied 2-4-2 engines. 
Small slope backed tenders were made from 1946 to 1958 and use mostly on switcher and small engines like 244, 1060 235, 1061, 1625, 1656 and more. They were 5 ¾ inches long. 
A typical square freight type tender (7 3/8 inches long) was made from 1953 to 1965 and was used on 233, 2018, 2029, 2037, 2055, 2016 and many others. 
A small tender was made for the General 4-4-0 engines from 195 to 1962. It was 5 ¼ inches long. 
I left out the sheet metal tenders (6 inches long) also called “coffin tenders” for reasons obvious from their appearance. They were based on prewar 1936 tenders. They were produced from 1946 to 1952.
I think most larger, longer locomotives (such as 671, 681, 773, 2020, 2046, 2065 etc.) look better matched with the larger Pennsylvania streamlined style tender even though many also came with the New York style tenders.
Almost all these tenders Lionel made were coal tenders. There were a few fuel oil tenders that were modified from the small streamlined tender, with the top coal load being replaced with a smooth top and hatches to represent a fuel oil tender.
Some of these fuel oil tenders were made for sale in cheaper sets and had a noise maker on the bottom that rolled on the center rail and made a low rattle noise. That is it hanging down in the middle of the car. These rollers can be pried out and the side pieces cut off to make a presentable fuel oil tender. They are very light and need some added weight to allow better pulling of a few cars.
I recently got interested in fuel oil tenders when I converted an enclosed cab engine into a cab forward engine and realized a coal tender would not work. I made a coupling on the front of the engine to hook up the fuel oil tender.
Fuel oil tenders also create a need for a fuel oil storage tanks near the tender coaling stations so the fuel oil tenders can be filled also. Fuel oil tenders were used in parts of the country in the where coal was not plentiful in the postwar era but do not get as much attention as the eastern part and coal country were more often modeled. Coaling stations and coal loading and unloading and coal mining are more interesting operations than fuel oil operations also.
Fuel oil tenders also open up the use of a second flue oil tender after the first one next to the locomotive. This can be done on real railroads to extend the range of the train as only fuel lines need to be added to connect the two fuel oil tenders to the engine. On a model fuel oil tender, the front truck should be changed out with one that has coupling on it while retaining the coupling for use of tender also with an engine.
Charlie
Post Script: See a following post dated 9-21-2019 for How to build Homemade Vanderbilt coal tenders.
