Travel Permit with Train from Xining

Hello,

My husband and I will be traveling extensively around China in April and May. In June we'd like to take the train from Xining to Lhasa. I realize that we need to book a group tour to get the Travel Permit, but can we book just a Lhasa tour (4 days) and still be able to travel on our own on the train from Xining? Or does our travel on the train required to be a part of our tour?

Thanks,
Liz