1. In a railroad tank car for use in transporting a commodity at other than ambient temperature, having an elongated tank designed for support near its ends over conventional wheel trucks; the combination comprising: a. an inner commodity confining shell; b. an outer shell surrounding said inner shell; c. insulating material located between said inner and outer shells; and d. center sill means for supporting said inner and outer shells above said wheel trucks; e. said center sill means including a pair of Z-sectIons secured together at their top flanges to form a channel shaped member having a substantially horizontal top surface, a pair of substantially vertical leg portions depending from the longitudinal edges of said top surface, and a pair of substantially horizontal bottom flanges extending outward respectively from the lower edges of said leg portions; f. said center sill means being received within a removed bottom section of said outer shell; such that the longitudinal edges of said outer shell, formed by the removal of said bottom section, are secured to said leg portions of said center sill means immediately below said top surface.

12. In a railroad tank car having an elongated tank shell, for confining a commodity, supported near its ends over conventional wheel trucks; the combination including: a. a removed longitudinally extending section formed in a bottom portion of said tank shell; b. center sill means for supporting said tank shell above said wheel trucks; c. said center sill means including a pair of Z-sections secured together at their top flanges to form a channel shaped member having a substantially horizontal top surface, a pair of substantially vertical leg portions depending from the longitudinal edges of said top surface and a pair of substantially horizontal bottom flanges extending outward respectively from the lower edges of said leg portions; d. said center sill means being received within said removed bottom section of said tank shell; and e. bolster assembly means for supporting the weight of the tank from the wheel trucks; f. said bolster assembly means including an arcuate saddle means which is inset and overlaps a removed portion of the lower sides of said tank shell.

15. In a railroad tank car for use in transporting a commodity at other than ambient temperature, having an elongated tank designed for support near its ends over conventional wheel trucks; the combination comprising: a. an inner commodity confining shell; b. an outer shell surrounding said inner shell; c. insulating material located between said inner and outer shells; and d. bolster assembly means for supporting the weight of the tank from the wheel trucks; e. said bolster assembly means including an arcuate saddle means which is inset and overlaps said outer shell adjacent a removed portion of the lower sides of said outer shell.

20. In a thin skin insulated railroad tank car for handling and transporting materials under other than ambient temperature conditions and wherein the tank car includes an elongated and enclosed commodity confining inner tank shell, an outer tank shell formed of metallic plates in the range of three-sixteenths inch to five-sixteenths inch thickness which forms an enclosure for and effects the confinement of insulating material that substantially entirely surrounds and supports said inner tank shell, and center sill means and bolster assembly means for supporting said inner and outer shells above a pair of wheel trucks; an improved structure wherein: a. said center sill means includes a pair of Z-sections secured together at their top flanges to form a channel shaped member having a substantially horizontal top surface, a pair of substantially vertical leg portions depending from the longitudinal edges of said top surface, and a pair of substantially horizontal bottom flanges extending outward respectively from the lower edges of said side surfaces; b. said center sill means being received within a removed bottom section of said outer shell; and c. the longitudinal edges of said outer shell, formed by the removal of said bottom section, are secured to said leg portions of said center sill means spaced a short distance below said top surface.

23. In a thin skin insulated railroad tank car for handling and transporting material under other than ambient temperature conditions and wherein the tank car includes an elongated and enclosed commodity confining inner tank shell, an outer tank shell formed of metallic plates in the range of three-sixteenths inch to five-sixteenths inch thickness which forms an enclosure for and effects the confinement of insulating material that substantially entirely surrounds and supports said inner tank shell, and center sill means and bolster assembly means for supporting said inner and outer shells above a pair of wheel trucks; an improved structure wherein: a. the annular area located between said inner and outer shells is encircled in spaced relationship with a plurality of inflatable bag means so as to compartmentalize said annular area; and b. at least the bottom portion of the compartments located adjacent said bolster assemblies being filled with a higher density insulating material than the other compartments.