Former publican Mick O'Neill has made a dream start to his training career at Randwick, toasting Mr Spin's success after he defied a heavy track to claim the Class Three TAB Highway Plate.

The 68-year-old secured his trainer's licence only six months ago after previously focusing on breeding, and the change of direction paid dividends on Saturday when the five-year-old gelding scored by 1-1/4 lengths after improving from eighth with 400m of the 1600m journey left to travel.

O'Neill, who counts fellow Coffs Harbour trainer Peter Roach as his mentor, paid tribute to the 83-year-old after Mr Spin - a $31 outsider - won for the third time in his 15-race career.

"Peter can take all the credit for this," O'Neill said.

"I've bred a lot of my own horses. I had too many and I decided to get Peter Roach out of retirement two years ago to give me a hand, with the intention that down the track I'd get my own trainer's licence," he said after jockey Rachael Murray saluted ahead of Trying.

Joint pre-race favourite Ruthless Agent was two lengths further back in third.

O'Neill, whose first purchase was the stallion Za-Im during his 18 years in the hotel industry, was barely stressed as he made his training debut.

"Someone said are you nervous? I said I'm not nervous because I've been here before," he said.

"I didn't know what to expect from the horse. He'd never run in the wet before. I had no idea, couldn't tell the jockey anything."

O'Neill was also unsure about the next move for Mr Spin, with the Highway series on hold until the end of The Championship.

"I could be back for another Class Three Highway. I'll have to go back and do my homework," he said.

Meanwhile, the Greg Bennett-trained Mister Marmalade was a late scratching after he received minor head injuries when the transporter carrying him to the track was involved in a car accident.

Greg Bennett Racing tweeted about the crash two hours before the race: "We have been involved in a bad accident .... passengers OK. Mister Marmalade minor head injuries."