Commuters using the Elizabeth Line could save money on their journey by tapping in and out at certain stations.
Anyone travelling into the city centre from the outer zones of the capital, such as Reading or Shenfield, could save money because of a ticketing loophole.
However, a trip from Reading to Slough costs just £5.90 while Slough to Tottenham Court Road costs £11.80.
The loophole exists as prices switch between Transport for London (TfL) fares and National Rail fares at stations further outside the capital, according to the Telegraph.
This allows commuters to make a saving of £10.80 if they choose to tap in and out at Slough rather than travel directly from Reading.
On the other side of the line, a peak time fare from Shenfield to Paddington costs £13, but commuters could save cash by splitting the journey up.
Travelling from Shenfield to Harold Wood costs £3.60, while Harold Wood to Paddington costs £5.50 – enabling a saving of £3.90.
Norman Baker from Campaign for Better Transport, a campaign group, told the Telegraph that the system was “complicated” and “confusing” and said “root and branch reform” was needed of the whole rail system.
The first train left Abbey Wood station at 6.30am, followed three minutes later by a similarly packed eastbound service from Paddington.
Around 130,000 people travelled on the line in the first few hours of Tuesday morning, according to TfL figures. The line stretches from Reading in Berkshire and Heathrow Airport in west London to Abbey Wood in south-east London and Shenfield in Essex.
The £20 billion pound project is the most significant upgrade to the capital’s transport infrastructure in more than 50 years.
A TfL spokesperson said: “Fares between Reading and Slough are set by GWR and remain the same as those which operated under TfL Rail.
“As with all rail journeys across the UK, there will be occasions where customers may benefit from split-ticketing or pay as you go fares through exiting and re-entering stations with their contactless card.
“Our pay as you go system calculates the most appropriate fare for your journey, including whether any daily or weekly cap would apply. Pay as you go with contactless can be used across the whole of the new Elizabeth Line.”