A rider car would have been a coach or a baggage coach for the maintenance crew. Most likely a P70 or a PB70 depending on the number of crew members. Rider cars were also found at the end of mail and express trains and provided lost cost tickets for passengers as the train had no amenities beyond a restroom and a water cooler.
The PRR was not unique in the use of rider cars on M&E trains, but for a clearance test train that is all PRR.
If you are looking for a good rendition of a scale P70 there are several examples available from GGD, Sunset, American Models, PSC, Alco Models, Walthers, and a few I'm forgetting at the moment. The GGD plastic P70 is typically going to be the best priced one and is one of the few available in 3 rail. The P70 was an 80' car.
If scale fidelity is not as important to you, any 18" PRR heavyweight coach from MTH, Williams, K-line, or Lionel can sub for the scale coach, but the biggest challenge for me at least is that the P70 rode on PRR designed 2D-P5 4 wheel trucks and all the 18" renditions come with 6 wheel trucks.
Hope that helps!