You can try widening the input pulse if you want to see it flat on top. The pulse propagates largely unchanged in shape down the line. This is an animated gif, sweeping along the time axis over the course of a few seconds. On the original outbound pulse, voltage and current have the same sign. On the return pulse, current is negative since it is going backwards through the elements, opposite to our assumed direction of positive currents in the inductors in the figure above.
Voltage is still positive, however, with respect to ground.
While we're at it, here are two more cool ways to visualize the same 3-d dataset. First we will make the input pulse wider with hopes of obtaining a clearly visible flat-top. Here are several widths. First, the original pulse - this is 8 timesteps wide, or 4 seconds wide. The response is not very flat topped.