The Texas Mountain Trail region is a great place for train spotting.
The main line of the Union Pacific parallels US Highway 90 and there are over 40 trains a day from mixed freights to containers. Amtrak operates once a day on the old Southern Pacific Sunset Limited route.
A great place to start is the town of Valentine. As this map shows it was once a major division point on the Southern Pacific. While there are only vestiges of the of the old yards, one can approach the tracks without obstacles. Cross from US 90 north into Valentine to reach the main line.
Another great location is the old Southern Pacific station in Alpine, where trains often stop to change crews. This is also a stop for Amtrak.
Also Marathon is a great place for train spotting. One can sit on the porch of the Gage Hotel, just across from the tracks and watch Union Pacific equipment pass day and night.