In CAPE's travel tips its there. Last month I also tried to book but rooms were full. RR is there at CAPE. See the thing is for sunrise you have to be at CAPE by max 0530 hrs. After that sunrise can happen at any time. My parents went there 2 weeks ago and they witnessed the sunrise at 0550 hrs. So my advice is just get this ticket extended by onboard TTE, you would hav to pay 20-30 rupees. Get down at CAPE, look for Retiring Room. If not go for hotel. You can get hotel in around Rs 500-600.