We're coming down from Western New York & Southern Ontario. Is this type of thing you can/need to purchase tickets in advance before we come down? Not sure if it's likely to sell out.

Not likely to sell out. But it is recommended to get the tickets especially if it's for the weekends oh, and the weather is nice.

i would suggest buy online... we bought at station directly, there was plenty empty seats in train, but it was April, not sure how much crowded its there in summers,

Buy them online and avoid the possibility of being disappointed.