sag wagon
n.— «A “sag wagon” will follow the riders to pick up any who have mechanical difficulty or find the ride too strenuous.» —“State Plans New Event on Bikeway” Commonwealth Reporter (Fond du Lac, Wisc.) June 16, 1967. (source: Double-Tongued Dictionary)
Further reading
What if, instead of being an inanimate object, a dictionary were alive? That’s the idea behind a lavishly illustrated new children’s book called The Dictionary Story (Bookshop|Amazon) by Oliver Jeffers and Sam Winston. This is part of a...