The speedy son of Starspangledbanner was slashed into 6-1 favourite (from 14) for the Coventry by Betfair and Paddy Power following his stylish Marble Hill win last month but connections have resisted the temptation of travelling to Ascot.
Explaining the decision, Stack said: "We were tossing it over and back and going through all the pros and cons and, in the end, we decided to go for the Railway Stakes. There is only ten days between the two races and we couldn't do both.
"Just with the way he has to be ridden, if you got drawn on the wrong side over there in a big field it would be very hard for him. Then there is the travelling aspect of it to take into account as well. I suppose you could say it is the safe call but we do hope to travel with him later in the season for something."
There are certainly no health or fitness concerns attached to Castle Star's omission from the Coventry and the trainer said: "He's great and he went very nicely in his work yesterday. We're very happy with him."
In the absence of Castle Star, the Wesley Ward-trained Kaufymaker is the new 4-1 favourite with Ladbrokes.
Ladbrokes: 4 Kaufymaker, 6 Gisburn, Masseto, 13-2 The Acropolis, 8 Dhabab, 9 Cadamosto, 10 Ebro River, 16 bar.