thank you for your post.
The SBB Mobile App has a specific setting which allows users to purchase tickets worth up to CHF 40.00 without further confirmation. Maybe this setting is active in your app?
If this is the case, the purchase done is legitimate, and you should have received a confirmation email as well.
If you have not received an order number nor a confirmation e-mail from SBB, it means that the amount was not charged definitely.
TWINT authorizations are not definitive debits, although they are displayed on the bank account. The authorizations are automatically released again. This can take between 7 and 14 days with TWINT. Once the amount has been released, it will no longer be visible on your bank statement. For this reason, we recommend that you always wait for the final bank statement.
If you have any other question, please contact our colleagues at the SBB Contac Center (0848 44 66 88, CHF 0.08/min), they are available 24/7.
I wish you a great day.