I'm still debating (takes years...) whether to use D2 or D3 rails. I have a brand new set of D2 rails with 30 links and new parts to make it up to 33, but I could use that set on my 3J. A new set of D3 rails could be a better match for the rollers but I'd need to swap out the sprocket for one with D3 segments (with the later pitch). The earliest model of D3 has the same pitch as the D2 (6.125) so those rails would work but the grouser plate holes don't line up and are a different size so a lot of drilling through rock hard plates would be required, vs. having to build up my existing plates - so lots of factors in the decision (plus it's not burning a hole in my pocket so I keep putting it off....)