What is a rail track bar size and material?

If i understand the question correctly, the bar size and it's material depends on the weight of rail that is bought...the weight of the rail is determined by a three foot section, there are various types of rail, for instance, a 115RE rail means that brand new that rail will weigh 115 lbs per three foot. The bar has to match the rail because different rail are different heights, weights, and overall, different measurements on a cross section