Jiří Zemánek and Martin Gurtner were asked by journalists from technological on-line magazine Technet to place a remote-controlled Christmas tree on-board a tram that lubricates the tracks around Prague. To control the lights, they used a Raspberry Pi to run a string of addressable WS2811 LEDs. The algorithm to produce the light pattern (which is very complicated) was open to modification by website users who could send their own ‘wave’ to interact with the programmed ‘wave’. You can read more here.