DCTA vice president Tom LeBeau says the slowdown is for safety reasons because of the extreme heat.
Dallas Area Rapid Transit says Trinity Railway Express trains, between 2 and 9 p.m., will also operate at reduced speeds this week.
Union Pacific Railroad on Tuesday afternoon notified New Braunfels police that a train was blocking a crossing due to heat-related problems with a section of rail. Traffic was rerouted as the line was checked. FM 306 reopened Tuesday night.