With the Northern Central a Confederate target during the Civil War, the tunnel was protected by members of the Pennsylvania Militia. Aided by the fact that the tunnel is not accessible from the local roads, the militia successfully prevented Confederate attacks. The increase in traffic on the Northern Central required a second track be installed along the length of the route between Harrisburg and Baltimore. This required the Howard Tunnel to be widened to accommodate two tracks, and this was completed in the late 1860’s. The tunnel has been refurbished or rebuilt at least three times in its long history and has seen active rail service since its opening in 1833, making it the second oldest active railroad tunnel in the United States.