If you have a paid plan with Xero, then you can access the Xero developer portal without an additional cost. If you do not have a plan with Xero, you can sign up for a 30-day free trial. Even after the trial period ends in 30 days, you will still have access to the Xero developer portal.

You can sign up for a 30-day free trial on Xero here and you can access the Xero developer portal from here.
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Please see here for more information on integrating Xero with Railz.