Sorry, gentlemen.  The Titanic engine room scenes were of an actual
ship, the last surviving member of the Liberty class.

  http://www.cinematographer.com/magazine/dec97/titanic/adabt/pg1.htm  --

"For the engine-room sequence, VIFX composited live-action actors shot
against green-screen into the real engine room of a working Liberty
ship, which Vision Crew Unlimited had transformed into a 1/3-scale
miniature of the Titanic's engine room (see accompanying on-line story:
Ship Building).

"The Liberty ship was a real ship whose engine type was similar to
that of the Titanic," explains VIFX founder Richard Hollander.  But
its engine room wasn't the right size, so we added people in the
appropriate scale to make it look huge."


  http://www.cinematographer.com/magazine/dec97/titanic/sb/index.htm  --

"Cameron wanted to dramatize the moment where the Titanic's massive
engines went into full reverse to avoid colliding with the iceberg, but
we didn't have time to build a big, bitchin' engine-room miniature,"
Jacobs explains.

"Instead, someone came up with the brilliant idea of shooting the
engine room aboard a Liberty ship, a real World War II troop trans-
port moored in San Francisco.  Its engines are very similar to the
Titanic's, but one-third the size, so they decided to shoot that real
engine room as a miniature.  The only way to sell that was to put
things in there that forced the scale.  We actually took out all of the
catwalks, gauges, dials and light bulbs in certain areas, picking our
angles carefully, and replaced them with 1/3-scale versions.  Then VIFX
composited people onto our 1/3-scale catwalks and stairways to give you
a sense that the engine room was much bigger than it actually was."

Larry Smith