Sub station circuit diagram

Yep your guess is wrong! In the old days the stepped down ac from 22kV or 11 kv was rectified by mercury arc rectifiers to produce the nominal 630vdc, these days the same job is done with semiconductors.

The positive and negative potentials are purely arbitary as the 630v dc source is earth free, i.e. neither positive nor negative is tied to earth directly. However, at the end of each main traction section the positive and negative traction rails are connected to the continuous running rail (which is bonded to earth) via high wattage high value resistors (usually of the order of 7.5 to 10 kilohms) to reference positive 420v above earth and negative 210v below earth. This arbitrary fixing of voltage enables earth fault relays in each main section to detect positive and negative traction earth faults and indicate them at the Network Operations Centre and Line Control Rooms, as an earth fault on one traction rail will drive the other rail to a full 630v positive or negative. Trains are unaffected by a single earth fault and it would take one on each traction rail in the same section to trip the circuit breakers.

A main section comprises multiple sub sections, for instance until the 1980s Section 1 was Baker Street to Amersham, Chesham, Watford, Uxbridge and Stanmore until we split it at Harrow-on-the-Hill to become Sections 1, 36, 37 and 38 on the Met and a separate section 1 on the Jubilee (IIRC).

From memory the entire Piccadilly line is just five sections, two it shares with the District (10 & 11 IIRC) and it has three of its own being sections 8, 9 and 34 although this may have altered since Heathrow T5 was added to the line.

Sectionalisation in this way assists traction earth fault localisation but it can be a nightmare especially when the earth fault is actually a faulty train, because the fault will travel! In my experience traction earth faults were usually due to discarded drinks cans (especially in the tube) or defective train shoes.