Is this the same battery as my Festool 55 Track Saw?

No, the Festool hybrid 18V sanders use a different battery format. However, both battery types can be used on your TSC 55's charger. Link to the Festool 18V "ERGO" battery that works with the sanders below.https://www.toolnut.com/festool-202500-18v-3-1ah-ergo-battery.html