Jury selection begins today in Towson, Md., in the trial of a former Conrail engineer charged with 16 counts of manslaughter by locomotive in Amtrak's worst crash. Sixteen people were killed and 174 were injured in the Jan. 4, 1987, accident. Rick L. Gates was operating three linked Conrail locomotives that prosecutors say ran a stop signal in Chase, Md., and crossed into the path of a 14-car Amtrak passenger train. Federal investigators say Gates, 33, failed to notice the stop signals because of drug and possibly alcohol use. Gates has denied smoking marijuana while operating the train, but he tested positive for the drug.