You're welcome
There's a cloakroom at Tivoli too... so you can leave your bags there if you need it (or in a safe at the central station). Both costs money, but you don't have to go to Amsterdam first
.
And about the trains... there are night trains between Utrecht, Amsterdam, The Hague, Rotterdam and some smaller places in between... Every hour there's a night train although it's kinda far to travel by night train from Utrecht to Rotterdam or the other way around, because you stop at Amsterdam, Schiphol, The Hague etc etc until you are at the end of the line in Rotterdam (2,5 hours or something). But you don't need to travel that during the night, so it'll be fine
. Just keep the timetable in mind so you don't have to wait for almost an hour
.
Good luck and hope to see you at the Frames gig in Tivoli! I'll definately be there (and at Damien's too, but that venue is a little bit bigger
).