BNSF Railway Co., one of the country's biggest consumers of diesel fuel, plans this year to test using natural gas to power its locomotives instead. If successful, the experiment could weaken oil's dominance as a transportation fuel and provide a new outlet for the glut of cheap natural gas in North America. The surplus, spurred by new technologies that unlock the fuel from underground rock formations, has sent natural-gas prices plummeting. That has prompted industries from electric utilities to tugboat operators to switch to gas. If freight rail joins the parade, it would usher in one of the most sweeping...