make sure all your services are on the same region and that you're using the private network when possible

To enable private networking , I just need to click on enable private networking right?Will application get impacted because of that or any configuration changes needed?

Private networking is always enabled, you just need to connect to your database under the private network rather than the public one (basically use the railway.internal connection string rather than the proxy.rlwy.net one)

Also make sure your services all have the same region

any example because its not happening!

In my case node app has replicas 1 in singapore region, other frankfurt.
mysql is in frankfurt.
Used .internal host while connecting but it throws exception.