Which train is that? Route, date and departure time would be handy.

Some trains, like the Eurostar and Thalys, have a limited number of pass holder seats, which means that you can buy normal tickets after the pass holder seats are sold out. But if you give your exact travel details we'll tell you if that is the case.

Hi, it is for Eurostar on Monday, May 22nd, from London to Paris. On the Eurorail site no trains come up for this day, but on Eurostar all the times are available. For curiosity I checked if it was because it’s a Monday and noticed there’s spots available for this coming Monday, May 15th. So this lead me to think potentially additional spots open leading up to the date. 

Another note to add about this date, when I look it says “No trains available” and I after clicking through I see the next available train is Tuesday and see all the trains are Sunday, May 21st are sold out. 
 

If May 22nd, doesn’t even say sold out, does that mean no times were ever available for this day for eurorail pass holders? 

There are no pass holder seats available on the 22nd of May. There are sests in the morning of the 23rd. 

Sometimes there are more seats available between London and Lille, but on the 22nd there are no pass holder seats to Lille either. 

Hi, do you know why that would be? Or some days fully unavailable for pass holders? It’s too bad because this train route was the main reason I purchased a eurorail pass. I was not able to see this day was unavailable until I bought a pass and I had a pass number so I could search to reserve.

Although too late for you there is a site that allows you to check availability for pass holders. 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

You do not require a pass to check availability.

Yes Eurostar sells out weeks in advance due to the quota (unfortunately)…

Your other options are :

- (expensive) regular ticket

- Flixbus 11:00 - 21:10 / 12:30 - 22:55 38$

- train to Dover, 25-min walk to the harbour, P&O ferry Dover - Calais 30£, train to Paris (would also take a full day). Leave London at 7:07 and arrive in Paris at 17:32

Eurail website only shows availability when you've bought the pass but no need to have one in order to use other websites