Yeah, I don't really know 100%, but in looking at things, looks like the individual engines all are LionChief "Plus" which for all the ones I checked, they have speed control. But the RTR sets just have LionChief, an don't.
Don't know if this is what the "Plus" is to mean, but anyway.
Can say that for my set, the Victorian Christmas set. Which is basically a re-sticker-ed Hogwarts, that I ran it at 18V at full throttle, an it barely made it once about 70% around a full 180 curve of the O-36 track it came with, an it flipped right over an completely off the track. Didn't derail, just rolled sideways right off from centrifugal force. I think all in all this engine with its fairly high wheels an other designs, probably has a high COG.