Tips on the best means to venture Sri Lanka

Hello,
It is a pleasure to know that you like SL so much. Here you can get an insight about the Main line 
https://en.wikipedia.org/wiki/Main_Line_(Sri_Lanka)
https://www.youtube.com/watch?v=s8VNJ88AFWw
I have heard that it is a bit hard to get a seat some periods of the year so you may can contact the railway and make your reservations in advance.
https://eservices.railway.gov.lk/schedu … on?lang=en

Five days is not enough time to see the country so it will be a bit stressed journey, Anuradhapura is a fantastic old town to explore with a cykel. First night in Colombo, second night in Anuradhapura, third night in Kandy, from Kandy you catch the Main Line to Ella and back to colombo and maybe afternoon before you depart you will see little bit of Negambo. Sometimes you may have to change from train to buss to catch the train again. I hope someone who livs in SL will help you. Have a nice journey!