simulatortrain
jointed freaking rails
I am getting mixed messages as I read that they were old FM Trainmasters that were cut down into slugs. So they could have used multiple engine types and classified all of those boosters into the same class, so since multiple people are correct, then who ever wants to go next feel free to. (That is if you answer the question and got it correct and those people are, SimulatorTrain,HauntedClipp, and Jcurran.)
The RP-F6Ys were from Trainmasters, the RP-E6Ys were from RSD12s. Easy to mix up.