The Melbourne Cup is no certainty for Silent Achiever.

Roger James says the multiple Group One winner faces "D-Day" in Saturday's Turnbull Stakes, with the trainer reserving his decision until after the 2000m feature whether she heads to the Caulfield Cup or weight-for-age Cox Plate.

"The Cox Plate is probably more my leaning now that I've seen the weights for the Caulfield and Melbourne Cups, but no decision will be made on that until after Saturday's race," James said.

James says the mare has some enormous statistics to defy should he elect to run her in the Melbourne Cup with 56.5kg.

"We're taking one race at a time. With the weight she's got in the Melbourne Cup, I was pretty excited about getting there with her but I'm not quite as excited about it now," he said.

"And we have the autumn to think of. The Melbourne Cup is not a certainty."

Makybe Diva is the only mare to have carried more weight to win the Melbourne Cup, shouldering 58kg in 2005.

James noted Makybe Diva had won two Melbourne Cups, a Sydney Cup, a BMW, Australian Cup and Cox Plate before she won her third Cup under the 58kg.

And he said with the Caulfield Cup weights to be raised 2kg bringing Silent Achiever up to 56.5kg, history was also against her.

"I saw where you had to go back to 1942 since a mare carried more than 54.5 to win it," James said.

"So the odds are stacked against us a bit.

"If she happened to go to a Cox Plate and win it, we might look at completely different options."

This year's Melbourne Cup weights were higher than usual without what was considered a natural topweight.

"I've got enormous respect for her but she's a good mare without being a great mare in my book," James said of his stable star.

"The great mares are your Makybe Divas and your Sunlines and those sorts of mares.

"We beat some pretty nice horses in the autumn in the Ranvet and BMW and those races are going to be there again but those horses aren't. And sometimes smaller fish are sweet, and the Ranvet and BMW are not exactly small."

James said Silent Achiever had a good blow after a solid 1600m work-out on the Flemington course proper on Tuesday in company with Lets Make Adeal who shaded her on the line with the pair running slick time over the last 400m.

He expects Silent Achiever to improve markedly from the gallop ahead of Saturday's potentially campaign-defining race.

"I always said that Saturday's run will be pivotal on her preparation and that's still how I see it," he said.