Railsort has no account and no sign-up. The name you choose, your levels, stars, scores, streak and records are stored on your phone. If your phone's own Google backup is on, it can carry them to a new phone.
Every level and every mode plays in airplane mode. The daily challenge is made on the phone from the date.
The network is used for ads, served by Google AdMob, and for the optional purchase, handled by Google Play. In Europe, a consent form asks your choice before any ad; if you refuse, Railsort requests no ads. Privacy & ad consent, in Settings, lets you change your choice.
The details are in the privacy policy.