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.
On the flip side, if you loved your job, your coworkers, your boss, the cappuccino machine, everything about the place, don't write page upon page waxing rhapsodic about all that you're giving up by leaving. A brief, heartfelt compliment to the company, or a thank you, is fine.