The rail in the Pilbara is all AS68 kg/m which is a direct evolution from 136 RE as used in class 1 railways in the UK. BHP started calling it 68 RE until it was included in AS 1085.1 supplied to Australian requirements. It is generally supplied in head hardened these days, Hamersley used carbon on tangent tracks but moved to fully HH in recent years. Look out for through hardened and micro alloy rails. This section is a world wide standard and all rail mills are capable of producing it. It is rolled to an incredibly high standard. Standard length was 25m for years based on ship hatch size though most mills can supply longer. Rails are shop welded to 400m (FMG is longer, I think 500m) and all field welds are flash butt.
Sleeper design and spacing meet requirements for 40+TAL.
The ruling grade in the Pilbara is 0.33% and vertical curves are designed for the speed. The climb out of the Fortescue Valley is 0.55% on the BHP and a new route has been designed.
Rail/ wheel profile design is a black art and whole conferences study just that From memory, BHP and Hamersley were just different enough to prevent sharing tracks.
In essence, as above, the railways were built in greenfield, the original lines did not have the technical inputs available today. Over 100 alignments were studied by FMG. Flood mapping, heritage surveys and geology are in-depth inputs.
That covers part of the below rail, I'll leave others to describe how train operation is controlled to optimise operation. Continuous measurement and condition assessment is vital, one overloaded wagon or one wheel flat can cost millions. One train path lost is in the order of $2M lost sales so costs of raising standards are not significant on that side of the ledger.
Twiggy has certainly pushed the envelope from BHP which was regarded as the world benchmark in heavy haul.