I have not read something you describe or seen any errors in Apple Pay with my device or others in my family. You probably would have been better served by starting your own thread and describing your problem in detail, rather than adding onto a 2-year old thread.
What exactly are you doing when this seems to be appearing on the screen? Are you watching the screen that you know it is appearing when you are not near any NFC? If you have any rewards cards, they do not require that you double-tap the home button always, just bring the device in proximity, but many rental cars have NFC for additional purchases. Did you inquire with the rental company? What and where are you seeing this on a train?