In 1863, the Union army extended the Nashville and Northwestern Railroad to the Tennessee River, creating a reliable and secure supply line between the Ohio River and Nashville. Construction began in the fall of 1863 under the supervision of the 1st Michigan Engineers, with several thousand conscripted African Americans forming the bulk of the workforce, while companies of the 12th and 13th United States Colored Infantries guarded the laborers from raids by Confederate troops and guerillas. After its completion in May 1864, the railroad served as the major supply route for Union armies south and east of Nashville, linking supplies from Paducah, Kentucky, on the Ohio River at the mouth of the Tennessee, by steamboat up the Tennessee River to Johnsonville, then by rail to Nashville, where they were shipped to Gen. William T. Sherman's army in Georgia.