The Houston City Council, on Monday February 17, 1840, passed a resolution calling for the designing of a city seal. The seal was designed by state senator and former Mayor of Houston Dr. Francis Moore, Jr.

The center of the seal has the lone star, symbolizing the new nation in the west. The locomotive symbolizes progress, and the plow symbolizes Texas's agriculture; the seal symbolizes the rails transporting Texas's crops.

The reason why the train is featured in the seal is because of Houston's historical role in the regional railroad system; Houston served as a major railroad hub for many years, with tracks from more than a dozen companies crisscrossing the city.

https://en.wikipedia.org/wiki/Seal_of_Houston
https://www.houstonpress.com/arts/houston-where-seventeen-railroads-met-the-sea-6580406