Question about Brussels Midi train station
 

First timer in Europe. Am pre-purchasing train tickets for a day trip from Paris to Bruges, and I have to change trains at Brussels Midi station. I'm trying to figure out how much time to allow for the transfer. My choices are either 20-min or a 50-min between arrival at Brussels and departure for Bruges. Is 20-min enough time to find the correct train, or should I allow 50 min? Also, do you have to go through customs when changing trains?

The trains from Brussels to Brugge run every half an hour and you cannot reserve seats, so you just get on the next train. There are no customs or passport controls between France and Belgium, so you'll get off the train from Paris, go down an escalator to the station concourse, look at the indicator to find the train to Brugge, then go up another escalator to the appropriate platform. Note that the train to Brugge does not terminate there, so its final destination will be different - usually Oostende or Blankenberge.

No customs between France and Belgium.

No formal customs but nevertheless French police do come on these trains and spot check luggage for contraband - i've seen it several times - they single out young folks suspected of coming from Amsterdam - i actually saw one bust. But in Midi no no customs.
If you want to avoid the often more expensive Thalys train the route via Paris-Lille-Bruges is about exactly the same time and could be cheaper.

20 minutes is fine.
If you miss a train, then catch the next one in 25 minutes.

Thanks for the replies. We've already purchased the first leg of the trip to Brussels via SNCF but got the PREMS rate so it was reasonable.

From what you all are saying, it sounds like we could just wait to purchase tickets for the train to Bruges at Brussels Midi, correct? There would not be a problem with availability?

Yes, No.

IF you want to buy the tickets at Brussels Midi, go to the "service interieur" section. However, if you get there and there's a big line and you see a train to Bruges is coming in the next couple of minutes, you can buy your ticket on the train. Just find one of the conductors (you will see them on the platform by the train) before you board or on the train as soon as you board and tell them you didn't have time to buy a ticket beforehand. They'll sell you your ticket on board for a very small extra fee. If you don't tell them before you board or immediately after the train leaves, however, you could get hit with a more substantial penalty.
I do this often. Sometimes the lines at the ticket counter can be pretty long.