You don't always have to put folds or ribbing on areas like rails as it is unreasonable considering the shape of material to begin with. That is why they use larger beams where the cross sectional area is larger. They only use folds and ribbing on thinner material to keep weight down.
When you say the welds should "only hold it together instead of taking all the stress'' isn't right, because you have just created a joining point. This then becomes a section where the force will be transferred through along the beam/material you have just welded in. The material will either be in compression or tension and the joints then have to hold that section together and not fail from the stresses that the car is then put through that joint(weld). The metal also needs to have enough strength and be tough enough to withstand these forces and not fail.
Simple materials engineering.
What you have done will hold.