There are two main classes of railroad locomotives: long-haul and short-haul, or switcher, engines.

Many locomotives in the Houston region used for switching and other short-haul purposes have been replaced with cleaner engines with funding from the Texas Emission Reductions Plan (TERP).

There have been some complaints from others that railroads get a lot of funding from TERP, but do not pay much into this fund as do other classes of grant recipients. In response, railroad officials have commented that they do pay vehicle registration fees and some other fees. Furthermore, the railroad officials say that they provide a greater cost-share than other types of projects.

Railroad companies have also commented that they are on a path to upgrade their equipment ahead of expectations, which will result in emission reductions. In general, they believe that the amount of air pollution they release into the air is overstated by the emission inventory models.

However, the emission inventory models are highly simplistic and do not reflect actual measurements of emissions in the Houston region, nor do they include very specific data about the nature of Houston-area operations. So these emissions inventory data may be quite inaccurate.