Whenever a Eurostar gets into Brussels, and especially when one gets into Paris, there is always a mad scramble for the metro ticket machines, and if you weren't in the first coach then a very long queue...
A couple of years ago I did manage to buy Paris metro tickets from the Eurostar information desk in London before I left, as described here. (It was at the information desk on the far right after security and passport checks). I seem to recall they even sold me the tickets in sterling at the day's exchange rate!
I've subsequently recommended this to a friend, who wasn't able to buy the tickets from the information desk, and instead had the long queue at the metro ticket machines on arrival.
Does anyone know if Eurostar are still normally selling metro tickets from the information counter? And/or does the buffet on board sell them? (I think I've once heard an announcement about that)