ResNets, DenseNets & UNets

The most important question is how to train the deep convolutional networks? And quest to answer this question led to the invention of the ResNets. As per the traditional approach to train deeper convolutional neural networks, we increase the number of layers. And as per our knowledge on convolutional neural networks, the below should happen -- With the increase in the layers, the network should have low training error. But this was what didn't happen!