If you already tested this by moving the Russians out of the way then your 'turn 39' answer is likely correct. It's been awhile since I posted this question &Schnurri wrote:In my hotseat I cleared the way of Russians and let the Panzers get there as quickly as possible and was only able to rail at the end of Sept. Sea invasion would have been 1 turn faster I think.
I may just have based the calculation on the developers rail conversion speed notes. In that case my error might have been assuming the conversion starts on the invasion turn, when it actually would start on invasion turn +1. If I find out anything else in a re-test I'll post it. Either way, feel free to continue the game Schnurri

Regarding faster rail using a sea invasion - pretty sure that is not how the rail conversion feature works. I'm basing that on paradrop tests (similar to a sea invasion in that you reach hexes far behind the front line) where the rail conversion speed remained the same, and those paratroops had to wait a long time for rail access even when a land supply path reached them a turn or two later.