I am taking the Eurostar from London to Paris Nord. Are there ATMS in the train station or should i get some euros before I leave London?

Yes, there are cash machines at Gare du Nord.

If you can at all avoid it -- never get currency in a country where it isn't used.

Don't buy € in the UK, don't buy £ in Paris or Brussels, don't buy $ outside the US.

But since there are ATMs everywhere in Paris including at the station, that isn't a problem.

And if there is a long queue at the ATM -- there are others just outside the station across the street.

There's a battery of six BNP Paribas ATMs directly across from the station, too.

"don't buy $ outside the US."

????

Exchanging cash for USD in the US is an expensive and painfully slow process.

If I needed cash and wasn't using an ATM I'd buy USD in advance