You can take the warp out of polystyrene based plastic. You will need a bowl of hot water, and some kitchen tongs. I'm assuming you are referring to the Airfix model wagon.
Bring the water to the boil, but not boiling IE, very hot. Immerse the cast plastic polystyrene part for 15-so seconds, and remove with the tongs. Gently bend the affected part back to true, and allow to cool.
You will need to satisfy yourself that this treatment will not materially affect that part which you aim to modify. I will not, repeat not accept responsibility for posts on here, because their prize locomotive has disappeared down the plughole!
Experience will tell on how long you immerse the hot water, and to what extent you can bend the shape. Remember that the thicker section of material-any material- will take longer to accept bending. If you're trying to bend any material beyond its design parameters, it will snap. Remember also, that it might take several immersions before you reach the desired shape.
Sometimes with high-speed plastic injection, the operator will have a cooling block, to allow the impressed part to cool off whilst remaining the shape.
Remember:- Safety first! Despite what you've heard, steam will hurt! Been there, done that, got the T-shirt.
Happy modelling!
Ian.
Edit: Thanks to Mr Hayter for pointing out my error, and a reminder to self to proof-read my own posts from time to time...