Here is one link of a PRR X29 box car:http://www.billspennsyphotos.com/apps/photos/photo?photoid=75348380
Also, notice, the extra attachments of PRR's REA box car, as Rusty Traque's photo. shows and as, Keystone Ed points out.
Now, another link showing the transformation into a PRR X29B box car, for PRR MS P I about 1946, or 1947, as shown: http://www.billspennsyphotos.com/apps/photos/photo?photoid=121415257
Notice, if you can, the size differences between the X29 and X29B cars.
The X29 car is lower and slightly narrower, than the X29B car.
Ralph