I don' t know how many iOS developers share with me the passion for the model railroad , but it worth a try.
I've release on the app Store some days ago iThrottle, an SRCP client able to control a list of locos over your layout.
You can find more info on my dev site :
ATDev IOs Applications | ATDev Games and Utility for IOS deviceATDev IOs Applications
The app is not standalone and use the SRCP protocol to drive your locos on the layout.
Link to AppStore :
iThrottle per iPhone 3GS, iPhone 4, iPhone 4S, iPod Touch (terza generazione), iPod touch (4a generazione) e iPad nell'iTunes App Store
Screenshots :