Welcome aboard. The first question is what scale are you going to build in - HO, N, Lionel, or what? This makes a difference in terms of what kind of rolling stock is available.
The first thing you should do is get a copy of
America's Fighting Railroads from
http://www.motorbooks.com/ProductDetails_6932.ncm. This is a great book full of pictures of troop trains, how military equipment moved, motive power, and how the railroads moved goods and people across the country in the face of shortages of men and materials.
Troops moved in regular passenger cars up until the beginning of 1943, when Pullman built 2400 troop cars based on the standard 50 foot boxcar. Some of these are available in model form, like this one:
http://www.walthers.com/exec/productinfo/932-4151, in HO scale. These cars were used through the Korean War and many were sold off as surplus to the railroads and converted to baggage and express cars.
The most common train during the first two years of the war was tank cars. Lots of tank cars. We had almost no pipelines and all oil and gas was carried by ocean going tankers. The German submarine attacks along the east coast put an end to that and railroads were called on to carry all the gas and oil from refineries to depots. The railroads pressed every tank car they had in service had many more built to handle this tremendous load. Solid trains of tanks cars were probably the most common type of freight train up until about 1944, when oil and gas could once again move by ship.
Military equipment like tanks, jeeps, and trucks moved by flat car so you'll need lots of flatcars. Some were owned by the US government later in the war but just about any kind of flatcar that had the capacity was used to transport military equipment from factories to port of embarkation.
On top of all this, the railroads still had to carry normal freight, like meat, fruit, canned goods, and raw material commodities like wheat. These were carried mostly in boxcars or refrigerator cars. There were few specialized cars like the covered hoppers we see today. Coal was another huge commodity to be moved so solid trains of coal hoppers were also common. If I was only going to buy three types of general service freight cars for the WWII period, they would be 40' boxcars, 40' ice refrigerator cars, and coal hoppers. There were other types of cars in use but these three types of cars were by far the most common.
That's a little history. Translating this to a layout is both challenging and fun. You might want to read a beginners guide written by one of our members, Chip (Spacemouse) at
http://www.chipengelmann.com/trains/Beginner/BeginnersGuide01.html. It will only take five minutes but makes you think about some basic questions for a layout. Once you get a vision and a plan, you'll need a lot of other books too but we can talk about those later.
