Combined with his previous experience Montague continued to learn his engineering skills by apprenticing with Judah.
On February 12, 1862, Montague went to work for Judah now the Chief Engineer on the Central Pacific, helping with the location surveys over the Sierra Nevada (U.S.).
Montague was appointed Chief Engineer of the Central Pacific Railroad in 1863 after the death of Theodore Judah.
He was the engineer charged with directing the locating, designing and building the western section of the transcontinental railroad that linked the west and east coasts of the United States, that linked Promontory Summit, Utah, to Sacramento, California.
Montague was assisted by his fellow engineers Lewis M. Clement and James Harvey Strobridge.
Montague married Louisa Adams Redington in San Francisco, California, on February 13, 1868.