Source: Armored Trains, by Steven J. Zaloga. 2008. Osprey Publishing, 443 Park Avenue South, New York, NY 10016, Page 5.
"The American Civil War also saw significant use of armored trains in combat. The first American armored train was built to patrol the railways north of Baltimore against Confederate saboteurs, the earliest example of armored trains performing their classic role of antipartisan warfare. In June, 1862, the Confederate commander, Gen. Robert E. Lee instructed the commander of his artillery to construct a railway gun wagon. A Confederate Navy officer placed a 32 pdr on a four-axle railcar protected by a wall of inclined steel rails and this armored battery was used in combat during the Seven Days' Battle near Savage Station (June 25 - July 1, 1862). The Union built a larger version in 1864 using a Parrott gun, and during the siege of Vicksburg in 1864 a 13-inch mortar was mounted on a railcar. These developments were noticed in Europe and during the Franco-Prussian War of 1870-71 the Compagnie d'Orleans built an armored train with two armored wagons for 140 mm guns, and used this vehicle during the siege of Paris."
I have received additional information from a colleague in New Bern. However, our efforts to prove the construction of a monitor car in New Bern has eluded us. Certainly, New Bern's large railroad depot and foundry could have support such construction.
Thank you for your interest and the information you provided.