I took a train north from Moorgate on Saturday. A very quick turnaround, the southbound train terminated on platform 8, and reversed back again after only a very short pause.
Stepping back is in operation all day at Moorgate; one train at off-peak times and two trains during the MF peak. Stepping back is whereby, after arriving at the terminus with train 1, the driver (driver B) exits the cab and makes their way to the rear of the train, operating a 'rear cab clear' (RCC) plunger on their way. Meanwhile, a fresh driver (driver A) is already waiting adjacent to the rear (now front) cab of train 1, who boards and livens up the cab once they receive the RCC alarm. The process then repeats with train 2 arriving, its driver (driver C) alighting and then our original driver of train 1, driver B, then takes that train back out.
Train 1 arrives with driver B and leaves with driver A.
Train 2 arrives with driver C and leaves with driver B.
Train 3 arrives with driver D and leaves with driver C.
And so-on.
Where two trains are stepped back the principle is the same but with an extra driver in the mix.
Train 1 arrives with driver C and leaves with driver A.
Train 2 arrives with driver D and leaves with driver B.
Train 3 arrives with driver E and leaves with driver C.
Train 4 arrives with driver F and leaves with driver D.
&c.
Stepping back allows for near-instantaneous turnarounds, uplifting capacity and limiting terminating trains to the physical track capacity (platform re-occupation times etc.) rather than how long it takes for drivers to change ends. Ordinarily drivers are permitted 4 minutes to change ends; clearly a 5 minute headway could not be sustained from a single platform without stepping back.