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.
How to write an excellent resignation letter. Whatever the language, the sentiment is the same: I'm outta here. However, as a professional person, you need to express that maturely and firmly__ideally, one that doesn't alienate anyone at the job you're leaving. I'm sure we all have that fantasy where we tell off everyone who wronged us on the way out the door, but that person in the fantasy/movie/TV show never seems to have to live with the consequences afterward.