The problem with the system above is there is an instantaneous 900 mm/s velocity imposed at the start of the transient step. This creates an infinite acceleration, which lifts the front wheels off the ground.
A more realistic load would apply a finite acceleration of say 9000 mm/s^2, which is a bit less than 1 G, that is still pretty high. If you apply that acceleration for 0.1 s you can get up to 900 mm/s velocity, then for the next time period, you can set the velocity to a constant 900 mm/s. The cart will travel 45 mm in 0.1 s while it is accelerating. There should be some considerably longer distance before the bump to travel at a constant velocity of 900 mm/s so vibrations in the cart die down before the front wheels hit the bump. That means some damping has to be defined for the transient structural solution.