On the website's homepage, a box reads; Engines due for release around July/August, Butch, Dart, Den, Belle, Flynn & Rheneas.
This confirms that Day of the Diesels characters Den, Dart, Flynn & Belle will indeed be released to tie-in with the special.
Butch's re-release comes a month after his starring role in the Season Fifteen episode, Stuck On You.
It is thought that Rheneas' re-release has something to do with appearances in future episodes.
Thanks to The Colonel for the alert.