What do you about rusty top rails on an above ground salt water pool?

Try to saturate them with a lubricant such as WD-40 or heavy grease. Work them back and forth if at all possible. Try using leverage if you can. If all else fails, you can cut the bolts with a dremmel tool or drill through them with a drill. Be sure to replace them with stainless steel bolts and turn them periodically to prevent them from freezing up. Make sure the bit is the correct size for the fastener.