The Teensy library does not follow the rules of all the other boards. I have not spend enough time to unravel the code to easy understand it. It looks to me that RX should be defined as 0 (or CORE_RXD0_PIN) and TX as 1(or CORE_TXD0_PIN) , but I have not tested it. I am on short leave the coming weeks so do not have the time to invest.