Atlas did indeed do a beautiful X-29. The original model was contracted for by "Middle Division". They also did the H21 PRR Hopper.
The X29 was done by Atlas in paint jobs for other roads that had very similar cars such as NYC, B&O and some eastern roads. The X29 design was actually proposed as the 1923 ARA recommended design. It was not specified by the ARA committee, but some members liked it enough to build versions.
As for the group of photos, many of those cars have been down recently. Lionel did a decent PRR round roof 40 ft boxcar such as in the NP photo. They suggested they would follow on with a 50 ft version but have not so far. MTH did the drop bottom GN Gondola shown. Atlas and MTH have produced USRA outsided braced (sinle sheath) boxcars. They are both very nice versions. Any car as shown without runnin boards, would not be steam era. Atlas has also done a 50 ft single sheath car similar to as shown in their Train Man Line. Thse are not as detailed as the Master Line, but still very nice scale size cars. There were done mostly in Western roadnames as shown above.
I would like to see a detailed steam era 52 ft gondola. We have the ATlas Train Man version and the Mth version, but these have details cast on and look slightly crude when compared with the detailed boxcars and tankcars coming from these importers.