The original Loughor viaduct was designed by Isambard Kingdom Brunel.
Trains first began travelling over the viaduct in 1852.
Major rebuilding work was carried out in 1909, and there was substantial alteration in 1981.
17 timber trestles supported the old deck across the estuary. The new structure has six new piers supporting it.
Five trestles have been retained in the estuary as a reminder of the old structure, four to the west, one to the east.
The improved capacity offers the opportunity for one train every twenty minutes.
The length of track redoubled between Swansea and Gowerton is 5.25 miles long.
This stretch of railway is due to open on Monday with additional services timetable to begin on May 19.