The two original guns were sent to Nevada for test firing a nuclear warhead in 1953. According to the attached article a total of twenty "Atomic Annie's" were built but only four now remain in various museums (one being at Ft. Sill).
Here is a link to the write-up and pictures of that car at Oklahoma Railway Museum's website. It has now been repainted with MKT colors. I'm glad I got pictures before they repainted.
MKT 1045 - Oklahoma Railway Museum