Based on the information provided in the map description, the statement that best summarizes the system of railroads shown on the map is:
The extent of the railroads in the South was very limited compared to that of the North.
This statement aligns with the description that in 1850, most railroads were concentrated in the North and that during the expansion from 1850 to 1860, significantly more railroads were added, particularly in the regions west of the Appalachians and north of the Ohio River, further indicating a disparity in railroad development between the North and South.