#4018 was re-positioned in the late 1990’s in preparation for a starring role in a major motion picture. The movie project fell apart before #4018 was tore down as part of the proposed restoration to running status. The timing was fortunate...had #4018 been tore down, and then the project terminated, it is doubtful if the museum could have raised the funds to put her back together and move #4018 back to the museum.
Update — the Age Of Steam Museum has moved to Frisco, TX, located about 45 minutes north of downtown Dallas. They have also changed their name to the Museum of American Railroad. The Bigboy was slated to move in August or September of 2012. I have not heard that it has moved yet, but I have heard that the Dallas location is no longer open to the public. As a result, you might have to do some hunting to find this locomotive until I can confirm that the move to Frisco is complete.
Update — as of December, 2012, Bigboy #4018 is still in Dallas. The move had been held up until after the Texas State Fair was done, and then all of the other rolling stock and buildings had to be moved. The Bigboy and the Frisco are the last locomotives that need to be moved. Crews serviced the roller bearings in #4018 and prepared it to be towed by the BNSF. The move is now expected in January, 2013.
Update — Bigboy #4018 was moved from Dallas to Frisco in August of 2013 by the BNSF. Prep work involved little more than servicing the roller bearings. From there, it was towed at 10 miles per hour behind a pair of diesel-electric locomotives sporting a Burlington Northern heritage paint scheme and a string of tank cars used as buffers. The Bigboy will be a feature attraction when the new Museum Of The American Railroad opens.