I've decided to replace Gordon the High Speed Engine with Henry and the Express.
I swapped these out because I need to buy a lot of new engines and stock for Season 4 as it is, (namely another Culdee, a Take Along Culdee, and a new style 199, not to mention a second Wooden and Take Along Skarloey and Rheneas for cab removal...) and to buy the Brio Intercity 125 to make Pip and Emma would have been too much. On the plus side, I scored a Sodor Mail Car from ebay in MINT condition! This mean I will finally finish Really Useful Engines with Mind That Bike, likely replacing Twins Passing Problem.
Who knows, I might do it as a stand-alone episode before I air Season 4... only time will tell!