RAILGUN is open source and any developer can build with and integrate the RAILGUN Privacy System into their project. You can check out the GitHub repo here.
For developers interested in building with RAILGUN, there is the Quickstart SDK for easy integrations. Cross-contract calls are supported natively by RAILGUN, meaning users can use their Private Balances with your smart contract anonymously through minimal integration work with the Quickstart SDK.
Developer docs for the Quickstart SDK are available here. For deeper collaborations, please send a DM to @RAILGUN_Project on Twitter.