I am a newbe in the world of model RR so I need all the help I can get from you guys.
I am building a WWII diorama in 1/72nd scale which includes a German flatcar caring a tank.
So I would like to know using Evergreen strip plastic,what size of,maybe,I beam,a T beam or would 2 L beams would work for the rails in this scale?
You guys have a better idea to make the rails in this scale any information would be appreciated.
Thank you.

Others may disagree but IMO railroad rail is such a uniquely shaped extrusion it is generally not worth the effort or expense to scratch build. Considering that multiple plastic shapes will be required to form the rail profile the final cost may be relatively high once completed.

I feel an excellent alternative is to salvage the properly formed metal rail from old used toy train flex track. Flex track usually comes in meter (39") lengths and is available in a number of weights (125 lbs, 80 lbs, 60 lbs, 45 lbs.) as well as various scales (N thru G scale). The plastic or fiber ties are easily broken off and the rails salvaged.

Please note real RR rail also comes in more than one size. Rail used on sidings is often of lighter weight than main line rail. Also branch line and narrow gauge rail is almost always of a smaller size/weight than main line rail. This saves on construction cost and materials consumed.

Since S scale is 1:64 sized, this rail might be a good candidate to use for your 1:72 diorama. However do keep in mind that most model train rail regardless of scale is a bit oversized when compaired to the real thing. Myself I would use HO rail (1/87 scale) assuming it is slightly over sized it should be nearly perfect for use on a 1/72 diorama.

Also HO scale flex track will be the easiest to find in the local hobby shops.

I concur with Micheal.

FWIW, Italeri, Hobby Boss and a company named Hunar Product make 1/72 track.