A part-time racehorse trainer raising three young children has pulled off a major upset to land her first city winner.

Thunder Road's victory in the Highway Handicap at Randwick gave Goulburn-based Emma Longmire a win to remember.

"This horse you wouldn't think he's a world beater and to actually be here and actually win this race is amazing," she said.

The win squared the city winners' tally with her identical twin sister and fellow trainer Lucy at one-all, but the Longmires were denied a quinella as Percivale dropped out to finish ninth.

Emma Longmire recently gave birth to her third child.

While juggling the two commitments can be difficult, Longmire said her love of working with animals motivated her to keep training.

"I just enjoy the horses so much," Longmire said.

While punters and bookmakers gave Thunder Road no chance as he drifted from $31 to $61 before the race, Longmire knew the heavy track and apprentice jockey James Innes' 3kg claim would keep him in the hunt.

"I was a bit worried before the turn and then when Jimmy started riding him I thought 'oh well - here he comes'," Longmire said.

Thunder Road overhauled his rivals in the final 100m to beat She's Invincible ($7) and Grand Rouge ($10).

A combination of immaturity and injury has kept the rising five-year-old to five career starts.

Longmire said the owners had been patient with Thunder Road - who they call Thunder The Wonder - and their decision to persist has paid off.