If 2nd class reservations are available, can we book 2nd class even if we have a 1st class Eurail Pass?

My husband and I are going to Europe for 1 month will no real plan. I was looking at the Thayls train from Amsterdam to Paris and see that reservations are hard to come by the closer you are to the date of departure. We will have a 1st class eurail pass. If a 2nd class reservation is available can we book 2nd class instead?
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