If the tractor sells for more than you paid for it — or, in your specific example, the boat you traded it for is worth a larger number of dollars than you paid

If the tractor sells for more than you paid for it — or, in your specific example, the boat you traded it for is worth a larger number of dollars than you paid for the tractor, at the time of the trade — then yes; that is the very definition of "capital gains". Just because the gain isn't in actual dollars, doesn't mean there wasn't a gain. If the gain is somehow denominable in, or reducible to, a number of dollars, then for tax purposes it is a gain. You realize those gains at the time of the sale or trade. That's when they're taxable.