Suffering from a sluggish market for its business jets and an uncertain environment for its regional jets, Bombardier Inc. abruptly replaced president and CEO Robert Brown with Paul Tellier, 63, a Bombardier director since 1997 and president of Canadian National Railway for the past decade. Brown, 57, became president and CEO of Bombardier in February 1999 after serving for nearly three years as president and COO of Bombardier Aerospace, a unit of Bombardier Inc. Tellier is credited with transforming the formerly government-owned Canadian National into an efficient rail operator. Tellier will begin his new responsibilities on January 13. For the past 18 months, Bombardier Aerospace has been beset by a significant drop in demand for business jets and has even curtailed production of several models. The company is also concerned about financially strapped airlines delaying or canceling orders for its regional jets.