There has been lots of debate around repurposing Conference Centre back to its orginal use as Union Station, so here is some additional information about the building, courtesy of David Jeanes. (click "Read More" to see the aerial view image)

He writes:

Do not dismiss this idea. If the LRT tracks come up the Canal in tunnel to a
point north of the Mackenzie King Bridge, before crossing the canal, much
becomes possible.

To guide your thoughts here is a site aerial view, showing the relation of
the former Union Station to the surrounding walkable destinations, to many
of which it could be linked, under Colonel Byu Drive and under the canal.
Majors Hill Park and the Byward Market are also within 400 m. The dotted red
line shows 400 m from a second entrance under the Mackenzie-King Bridge.

My son and I have given guided tours of the Conference Centre to over 10,000
people over the past six years at Doors Open. There is overwhelming support
for restoring it to public use and as a train station.

I have studied its architecture for over 20 years, was the first person to
make copies of the original architects linen drawings, and have participated
in all the consultations for its re-use as Canada's Sports Hall of Fame and
the Canada History Centre. I have presented some of these ideas to the NCC.

The key to redevelopment is that the tracks would be two levels below grade
(to pass under the canal), and not under the building, so there is potential
for a below grade mezzanine surrounding the building but above the tracks.
This is shown in yellow. On the Rideau-Sussex corner it could be permitted
to rise above grade, without heritage issues.

The Rideau Street wing has five floors of offices, several with private
washrooms and fireplaces, and several high-quality meeting rooms on the
ground and fourth floors, two with heritage interior elements. There are
several possible uses for these, including as the headquarters for national
environmental and other NGOs, currently housed in a low-quality building on
Nicholas Street.

The main entrance has a grand foyer and staircase, leading to the great
hall, which should be preserved for public purposes and given a heritage
restoration. This could be as a top-quality space for use by the Congress
Centre and National Arts Centre, which could both be linked underground.

The transportation circulation needs, including tourist information, OC
Transpo ticket sales, food court, etc, could be accommodated at the south
end of the site in the former departure concourse, which is a grand space
stretching from Colonel By Drive to the canal. The adjacent former railway
office wing, which is underutilized due to fire code problems and has few
heritage features, could house the elevators and escalators to the new below
grade mezzanines.

The other canal-level spaces both at the Rideau Street and south ends can be
adapted for higher quality restaurants, pubs, etc. This was the Sports Hall
of Fame plan.

Linking the spaces north and south of the Great Hall, while maintaining its
integrity for assembly uses, has always been a problem. The solutions are
via the new mezzanine along the Colonel By frontage (partly below grade),
and an open space below the floor of the Great Hall. The latter, with
14-foot headroom, was part of the hall's original air circulation, but is
now unused. It could be tied to the new mezzanine at the south ends (east
and west), and north by escalators up to the Canal and Rideau Street levels.

The government now has other options for conferences, including the old
Ottawa City Hall on Green Island, and was previously prepared to unload this
building on the City of Ottawa. A strong public demand could tip the scale.