The underpass, a precast concrete "box", would be a few feet above the current track level, and set back from the track far enough to allow for a LRT station on the far side (southside) of Somerset, should one be installed. There are currently no plans for such a station, this is just good planning to allow the space for one.
The new tunnel would be both a cycling path and would permit pedestrians and users of any future LRT station to walk under Somerset and directly into any future City Centre redevelopment (recall the site is zoned for 5 or more towers, up to 22 floors or so)(not that any redevelopment is planned soon).
The City centre complex follows a curve as it goes north (the curve, of course, is defined by the curvature of the railway tracks that used to be under the elevated roadway and further west and north of the building). The cycling path would follow this curve and come out onto City Centre Avenue just north of Albert street, from which cyclists would have easy access to the downtown (via Albert) or to the River (via the Preston extension). Eventually, should Jim Watson fail to block the LRT project, the path could run north along the tracks to Bayview Station and the River and eventually the Gatineau Hills.