There are at least some in the Paris subway, including one that went at 12 km/h but was decommissioned in 2011:
https://en.wikipedia.org/wiki/Moving_walkway#Trottoir_roulan...
https://en.wikipedia.org/wiki/Moving_walkway#Trottoir_roulan...
I do think a concept with parallel tracks moving at different speeds would have been easier to use and more reliable though. But it might not have been revolutionary/over-engineered enough to attract attention and subsidies.
Speaking of speed, in the Stockholm main station the escalators go faster than others I've experienced... But I don't know if they've adjusted the speed since my experience years ago.