Train travel between Naples and Pisa along the coast

I don't think you have that option.
The High Speed line Naples - Rome - Florence is well inland. In Florence you change for a local train to Pisa.
If you took the local trains along the line which followed the coast, that would be a lot slower, and the line would not be on the coast, most of the time it runs along the coastal plain a few Km inland and out of sight of the coast.
The best rail map I could find of Italy is here: https://www.italiarail.com/italy-rail-map

You can look up train times and prices yourself on: http://www.trenitalia.com/tcom-en
This will default to the fastest route, if you want to force a different route, search for example Napoli to Formia and Formia to Roma.

Thank you .Good information

Don

It is not the fastest, but there are several Freccia, Intercity, and Regionale trains along the coast from Rome to Pisa every day.