Complex sentence: The other modified class were from the 51xx series (ex 31xx) and rebuilt into the 81xx class and retained the number 2 boiler, but again pressed to 225 p.s.i.
Split into simpler sentences: The other modified class was from the 51xx series (ex 31xx). This was rebuilt into the 81xx class and retained the number 2 boiler, but again pressed to 225 p.s.i.