These are vague... are you available to train a replacement anytime, even after your last day? Are you saying you'll be too busy to help prepare someone else or that you'll be busy helping with the transition? As with everything else in your resignation letter, clear and concise are the way to go.
Asking You to Stay Beyond Your Notice. In this situation, you won't necessarily get the promise of more money or a better title__but your manager will ask you, perhaps as a personal favor, to stay a few additional weeks or months until the company is able to hire and train a replacement.