Here is the full list of all 19 Tumbleweed Express achievements.
Complete a contract
Achieve a three star rating in any level
Find a golden envelope
Return to the Warehouse
Defeat the leader of the Drillcar Bandits
Overcome a bitter rival
Interrupt the status quo
Achieve three stars on all Desert levels
Achieve three stars on all Badland levels
Achieve three stars on all Cliffside levels
Find all golden envelopes
Achieve three stars on every level
Complete a level without losing a single freight car
Use TNT to blow up at least two enemies simultaneously
Complete five contracts at the same time
Achieve maximum train length
Purchase an automated turret for all freight cars
Acquire every Caboose Turret Type
Fully upgrade a freight car