In a post by 01Salty on the SiF Forums over the Yearbook, a Day of the Diesels section details on the Percy at the Dieselworks set.
The Dieselworks is where the diesel engines go to be repaired. Happy Hook is busy loading and unloading cargo while the cable at the smelter sheds transports scrap metal across the yard.
Diesel 10 oversees all from his glowing red shed.
Happy Hook seems to be another new character and is already being compared to Old Wheezy from Misty Island.
It is not currently known if Happy Hook will have a face or not.
Thanks to 01Salty for the alert.