In 1958 Trinity Steel merged with Dallas Tank Company, which was also founded in 1933, and Ray Wallace became the new firm's president and first chief executive officer.

For a time the company profited by producing larger tanks that enabled it to enter the petroleum business and do steel fabrication for refineries.

In addition, to free up capital, it established an investment company to buy trucks and lease them back to the firm.

[3] In 1970 Trinity diversified with the acquisition of 153 acres (0.62 km2) of land adjacent to the Dallas-Fort Worth International Airport and in 1971 established its first real estate subsidiary.

By the 1980s two subsidiaries, Gamble's Incorporated of Alabama and Mosher Steel of Texas, manufactured structural products including materials for drilling platforms, highway bridge components, commercial-high-rise buildings, and other girders and beams.

The firm's marine subsidiary, Equitable Shipyards, produced LASH or Lighter Aboard Ship barges, riverboats for use by Hilton Hotels, and other craft for industrial uses.

Trinity produced completed railcars, including tank cars, covered and open hoppers, and gondolas to transport chemicals, coal, structural steel and other commodities, at locations in Oklahoma City, Oklahoma, and Longview, Texas, and held two railcar leasing subsidiaries.

[6] In 2001 Trinity Industries acquired the designs and production facilities of Thrall Car Manufacturing Company.

[11] In November 2018, the company’s Energy Equipment Group, Trinity Containers, spun off and formed Arcosa Inc. (NYSE - ACA).

In March 2012, Joshua Harman, co-owner of guardrail manufacturing and installation companies SPIG Industry and Selco Construction Services, filed a federal False Claims Act (FCA) suit against Trinity Highway Products, LLC.

Trinity manufactures under license the ET Plus System—a guardrail end terminal system designed by the Texas A&M Transportation Institute (TTI).

Trinity made the modification upon the recommendation of TTI which had successfully crash-tested the product, it was revealed in a series of investigative articles published by television station WPRI-TV in May 2014.

[34][39] Trinity conducted a series of eight crash tests[40] at 27-inch and 31-inch heights to conform to the prevailing standard for guardrails of this type per the National Cooperative Highway Research Program (NCHRP) Report 350.

[43] In March 2015, federal officials said the TTI modified design manufactured by Trinity Highway Products met safety standards during crash tests.

[34] In September 2015, The FHWA and the American Association of State Highway and Transportation Officials published a joint task force report titled, “Safety Analysis of Extruding W-Beam Guardrail End Terminal Crashes.” The report concluded that there are no unique performance limitations that can be attributed to the ET-Plus as manufactured, that there are real-world conditions that exceed the performance expectation of all end terminal systems, and that additional crash testing of all existing NCHRP Report 350-compliant end terminals would be irrelevant and uninformative.

[27][28] On October 23, 2015, Trinity Highway Products announced that it would resume shipping the ET-Plus "after meeting safety standards in crash tests" to fill orders as they come in.