Ella train was fully booked, what are the alternatives.

While unreserved means you might have to stand for the first few hours, passengers frequently get off at smaller stations, so you will eventually find a seat. Plus, standing near the open doors offers the absolute best views anyway.

If you refuse to stand, your best alternative is to take a comfortable, air conditioned intercity bus to Nuwara Eliya, stay the night, and then catch a shorter bus ride or a private taxi down to Ella the next morning.