Castellina Scalo?
Ciao 4wine,
If you're arriving by train from Pisa to Castellina, then you mean "Castellina Scalo" and not Castellina in Chianti which is about 12 km northwest. Which is your ultimate destination when you arrive in the area?
There is a bike rental in Castellina Scalo but not on in Castellina in Chianti.
Here is the adress:
Furgoncin C. di Rinaldi Remo
Via Cassia Nord, 33 Castellina Scalo - Monteriggioni
Tel. +39 0577 305015 - Fax 0577 305036
Just so you know, there are two bike rentals in Poggibonsi in case that turns out more convenient to where you are going.
Ciclofficina di Bettini
Via Redipuglia, 17/19 - Poggibonsi
Tel. e Fax +39 0577 936881
Ciclosport di Porciatti
Via Trento, 82 - Poggibonsi
Tel. +39 0577 938507 - Fax 0577 996732
Hope this info is useful, do let me know if you have any other questions.