The Trinity Railway Express warned riders about travel delays because of excessive heat.
At 3 p.m., the temperature was 104 with a heat index of 110. In such excessive heat, track temperatures can reach 140 degrees, which can easily warp or kink the track and put the train and passengers in danger.
To ensure safety, trains were slowed until about 7 p.m.
Temperatures reached a high of 106 at Dallas/Fort Worth International Airport on Tuesday.