I'm not sure what you are looking for; historical information about locomotive designers, or someone to design a locomotive today. If it is historical information you are looking for, Raymond Lowey (1894 -1986) is probably the "Industrial Designer" that most people would associate with locomotive design. He did not, however, design the mechanical elements of the S-1, GG-1, T-1, and RF-16 locomotives, he "styled" their exteriors, which contributed to their aerodynamic efficiency, and passenger appeal (which translated directly into revenue for their owners). These four engines are probably his most "publicly recognizable" projects; he contributed to many others.
Pennsylvania Rail Road S-1
PRR RF-16 ("Shark Nose")
Heavy Freight Consist
In 1938 another American Industrial Designer, Henry Dreyfuss (1904-1972), designed the Hudson J3 engine series for the New York Central Rail Road used on the 20th Century Limited New York - Chicago run.
If you are seeking information about the "mechanical engineers" that have contributed to the development of locomotives, starting from 1804, try this site.