Trainor, one of the best tight forwards in the Highlanders First XV competition, played a key role in the lineout. He was also mobile around the paddock and used his strength to score two of his side's seven tries.
King's has a strong pack, and loosehead prop Sidney Fidow also scored two tries. Midfield back Reece Te Pairi had his kicking boots on and landed six conversions.
King's led just 21-17 at halftime but added four more tries and 26 points in the second half.
Other players to impress were flanker Logan Garforth, hooker Joe Cairns-Thomson and fullback Taylor Haugh.
St Kevin's College won its second game in the top section when it came back strongly in the second spell to beat Menzies College 8-3 in wet and muddy conditions in Oamaru.
Menzies led 3-0 at the break but St Kevin's took the lead with 15 minutes to go when second five-eighth Siua Ngala burst through the defence for the only try of the game.
Brodie Flannery kicked a penalty goal in the second half.
Southland Boys' High School remains unbeaten with three wins after recovering in the second half to beat John McGlashan College 34-16 in Dunedin.
McGlashan led 16-8 at the break and it took the strong Southland Boys' team 15 minutes to take the lead in the second half.
Halfback Daine Lowery scored McGlashan's try, and first five-eighth Thomas Johnson kicked the conversion and three penalty goals.
This was a significant performance by a McGlashan team that has been beaten by more than 60 points by Southland Boys' every year since the Highlanders competition started five years ago.
Other players to impress for McGlashan were No8 Joe Mullis, flanker Rory Ferguson, prop Hamish Hendry and winger Sean White.
