There were two separate & distinct types/designs of T&P 50' RB insulated box cars. 

The original GARX 51550-51559 were built in 3-55 by General American Transportation Corp. and leased to T&P for about 5 years. The 10 cars were then bought by T&P in the 1st quarter of 1960 at which time they became T&P 3655-3664. These cars had Duryea underframes, horizontal riveted seams midway down the car sides, and Chrysler trucks. Branchline Trains offers a model of the GARX cars leased to T&P but with an incorrect conventional AAR underframe.

At the same time these 10 GARX cars were built (March 1955), the T&P at Marshall, Texas, started building what became a substantial fleet of 50' insulated box cars of similar appearance as the GARX cars but with 16-panel riveted sides (vertical seams) and conventional AAR welded underframes. Like the GARX cars, the T&P home-built cars had Youngstown Flush Sliding Doors (plug) fitted to 7'-7" clear door openings. From 1955 through 1959, T&P built 140 nearly identical cars as follows.

T&P 3500-3519, 20 cars, built ca. 3-55, Evans DF Loaders (DF).
T&P 3520-3554, 35 cars, built ca. 4-56, Evans DF Loaders (DF).
T&P 3555-3569, 15 cars, built ca. 9-57, Evans DF Loaders (DF "wings").
T&P 3700-3719, 20 cars, built ca. 9-57, Pullman Compartmentizers.
T&P 3570-3594, 25 cars, built in 1958, Evans DF Loaders (DF "wings").
T&P 3595-3619, 25 cars, built in 1959, Evans DF Loaders (DF "wings").

In 1960 T&P-Marshall built 35 additional cars, T&P 3620-3654, which were nearly the same as previously listed except with 9'-0" clear door openings and "Super Cushion" underframes. Inside they came with Evans DF Loaders.

In the T&P Railway book by Joe Collias, on page 145 is a builder photo of T&P 3505 from the 1955 group.