UP, and WP, all of them riding on railroad-owned flats. Portec's RailPac design, developed in cooperation with Chrysler, was placed into service by ATSF, C&O, C&NW, D&RGW, GTW, N&W, RF&P, SLSF, SP, UP and several other roads, with most riding on leased Trailer Train flats. These two designs were refined during the late 1970s and by 1980 fully enclosed tri-levels accounted for roughly a third of the in-service fleet, however fully enclosed bi-levels lagged far behind. Roofs were not added to all autoracks until the mid-1980s, as it took time for railroads with low overhead clearance routes to modify their bridge and tunnel clearances to accept them. Consequently, some roofless and even open tri-levels and bi-levels remained in service into the mid-to-late 1980s. The number of manufacturers offering autoracks declined during the 1960s, 1970s and 1980s. Several firms simply exited the field, including AC&F, Evans, Dana, and Darby, while others were acquired by other builders. Pullman, after supplying the very first production tri-level autoracks to the Frisco, abandoned the field to concentrate on the flat cars that autoracks were mounted on, only to return briefly in 1976–1977 to build a few tri-levels for the B&O and Cotton Belt (SSW). Pacific Car & Foundry briefly produced a few racks at about the same time, as did Thrall. Whitehead & Kales, one of two dominant producers since the late 1960s, was acquired by Thrall in 1981. Paragon, the other leading rack-builder, was purchased by Portec in the early 1970s, which was in turn acquired by Thrall in 1985. Paragon's autorack designs were sold to Greenville Steel Car, also in 1985, which was itself acquired by Trinity Industries in 1986. Thrall itself was eventually acquired by Trinity as well in 2001. Currently autoracks are produced by TrinityRail, Greenbrier, Johnstown America, National Steel Car, and the Union Pacific Railroad. Vert-A-Pac and Stac-Pac Both railroads and automakers wanted to eliminate theft and damage from vandalism and weather, thus reducing shipping costs. They also wanted to increase the number of vehicles carried per rail car for the same reason. Toward that end, in 1968 General Motors and the Southern Pacific Railroad jointly began work on development of a radical new rail car designed to carry the Chevrolet Vega, a new compact car being developed by GM. Known as Vert-A-Pac, the rail cars would hold 30 Vegas in a vertical, nose-down position, versus 18 in normal tri-level