Motive Power Recommendation

There weren't many diesels that pulled passenger cars for the PRR that will match up well with this set.  The main diesel passenger engines were ALCO PA's and EMD E series.  Both are longer than the cars in this set.  On the prototype side there were a couple of Baldwin RS units but I don't know that these have been produced in O by anyone, let alone in Legacy.  A good match would be a semi-scale GG-1 but I don't know if one has been produced with the Legacy electronics.

Lionel released a legacy k4 a couple of years ago, there are still some available at some dealers. These are super smooth runners and they have the whistle steam feature.

Legacy k4

While these are traditional sized cars they will still look O.K. behind the scale k4 as its not a terribly large steamer.

I pull the 15" railking madisons behind my scale k4, they look much better than 18" cars going around my O60 curves.

Lionel did also make a Lionmaster legacy t1 3 years ago or so. These are much harder to find but would be the correct "scale" to match the cars. They have the "whistle steam" feature as well.

 Legacy Lionmaster T1

The original run of t1's  offered 10 years ago occasionally come up on the secondary market.

TMCC Lionmaster T1

K-Line did a semi scale PA-1 in Pennsy passenger colors about 10 years ago.  These came with TMCC and RS but will be very hard to find.  Good luck.

I agree with Rick.  I have both the Legacy K4 and the Lionmaster T1 and both would look good pulling this set.  They are full featured and have great sounds, especially the T1.  The fictional, or not, tuscan K4 would also look good.

As mentioned the last released command semi-scale GG1's would also fit the bill.  While not Legacy they have full command control, great sounds and are Pulmore powered for good torque, coasting, and speed control (not cruise though).

As a fictional consist you could also use the PRR Legacy Mikado.  It would look good sizewise and has the great Legacy features.