on the bandwagon—participating in what seems likely to be a successful enterprise (supporting a particular candidate, movement, etc. in the hope of a personal gain):
- Everyone is on the bandwagon it seems, whether through action or just rhetorical agreement.
on the wagon—(coll.) abstaining from alcohol:
- Vince had a weakness: he was an alcoholic. For ten months of each year he stayed on the wagon; twice a year, regularly, he fell off, sometimes with doleful consequences on the job.
Note: The expression does not correlate in meaning with the phrase in the cart—(coll.) in difficulties or trouble:
- You’ll have to do what you’re bid, or maybe you’ll find yourself in the cart.