Note: I am looking for prototype photos of this exact PRR X29 version.
As others have posted this is a model of a 1937 AAR box car. I am not an PRR freight car expert but the MTH model is likely close (and certainly close in overall dimensions) to an early car from among the 4500 rebuilt into X-29Bs.
There are no prototype photos since the MTH model is not an X29. It's a boxcar, it's lettered for PRR but that is about where the similarity ends. The Atlas model is accurate.
Atlas O actually offered the TRUE & CORRECT PRR X-29 boxcars.
Those statements should be tempered with the knowledge that there were many, many original and rebuilt versions of the X29 over the decades.
Be advised that the Atlas X29 model is based on an early X29 with small dimensions. There were several later rebuilt versions that used old X29 underframes with new bodies of greater height and width. The link below is the best on line reference that I have found on the PRR X29 series. Look at the photo section and you will get an idea of how different a "X29" box car could be. The early X29s like the Atlas versions are about the size of a USRA box car while the X29D and X29E resemble much larger post-war AAR box cars.
http://prr.railfan.net/freight...spage.html?class=X29
Pardon my ignorance, but I thought MTH only offered PS-1 boxcars
The first box car model put out in the MTH Premier line was a 40 foot PS-1 although MTH never advertised it that way. MTH now makes about a dozen different Premier box car models. The 40 footers are the PS-1, 1937 AAR and USRA single and double sheathed cars.
To be honest, this is why I have chosen to liquidate my MTH Premier rolling stock collection and stick to Atlas O. Just too many instances where they take a specific class of freight car and incorrectly represent it with generic designs.
Atlas has put out many "kinda close" versions of generic models too. I know Hill Lines freight cars well enough to feel comfortable saying that most of the O scale Great Northern box cars that Atlas has offered over the last 15 years have the wrong roof and ends for the cars that they purport to represent. A few are just totally dimensionally wrong.
Whether its Atlas, Lionel, MTH, Weaver or others, all the manufactures put out dozens of close enough (for them) box cars. If you know your prototype and evaluate each model you can decide if it is close enough for you before purchasing. 
