Train locomotives passing through or just idling at Union Pacific's Roseville train yard represent the single biggest source of diesel pollution in the six county Sacramento region.

A new locomotive emission control system known as an anti-smog bonnet may offer some relief. It's supposed to be able to scrub away 99% of the sulfur dioxide and soot from diesel exhaust, along with most of the smog forming nitrogen oxides that trains belch into the air.

But whether it works as promised is only one of the railroad's concerns. UP also wants to know if the system will slow the movement of freight through the yard. Congestion at the yards  is a major impediment to the efficient movement of goods and passengers.  But those concerns must not be allowed to outweigh public health. The plume of soot rising from the Roseville yards stretches across 100 square miles some days, encompassing most of Roseville and all of Citrus Heights and Antelope.

Pollution generated at the yards increases cancer risk for 165,000 residents. If the anti smog bonnet reduces that risk, UP must find a way to use it. If the railroad doesn't act to protect the public, state and federal regulators must.

Ginger Rutland writes for the Sacramento Bee Opinion pages.