Honestly I thought that rather than envy it was shock and maybe worry?? For them to have been intimate and known eachother for so long and for Rail to never have seen him make that face it means that Kouichi is SPECIAL to him, and that kind of pride and soft peaceful smile looked a lot like love??? My theory is that he's worried that Dyne is in love with Caius's bride to be and if he were to be found out, or if he decides to act on it- It would absolutely get him killed