Abstract: Method of constructing a tunnel under an existing structure, such as railroad tracks, without disruption of the existing structure, i.e. without interrupting railroad service on the tracks and without requiring construction of expensive track run arounds. The method comprises the steps of boring or drilling a plurality of pipes in side-by-side relationship generally horizontally beneath the railroad right-of-way; if need be, reinforcing the pipes to assist in carrying incremental components of the loading on the tracks; incrementally excavating a small segment of the tunnel immediately beneath said pipes and placing tunnel forming members in the excavated segment in supporting relationship with the pipes; and repeating the last two steps until the tunnel, complete with tunnel forming-pipe supporting members, extends beneath the railroad tracks and right-of-way.