What are tail currents?
OK, I know this is old, but here goes. The tail current is the current at -55mV after the voltage step, here:



The current shrinks as the voltage changes, this causes a massive shift in the driving force for the ions either side of the cell membrane. Going from -125 to -55 mV for the largest step. As commented, the dense vertical bands either side of the step are due to fast capacitance readings, basically ignore unless you want to go down a whole other avenue.

The reason for the tail currents is some channels remain open and close gradually after the step ends "tailing" off. We can't really say what channels these are as we know nothing about the recording conditions/cells involved. However these currents are large, so most likely to be sodium/potassium.