Original: Just beyond Heathrow Terminals 1, 2, 3 tube station, the line goes into a new section to serve Heathrow Terminal 5 tube station, which opened in March 2008.
Simplified: Just after Heathrow Terminals 1, 2, 3 tube station, the line will go into a new section to serve Heathrow Terminal 5 tube station, which is being built.