The £50 million six-week project began on 18 July and involves lowering 10km of track through Dundas Aqueduct, Box Tunnel, Middle Hill Tunnel and Sydney Gardens, along with the installation of 11 new sets of points.
By lowering the track, room will be available for the overhead line equipment needed to power a new fleet of longer, faster, quieter and greener electric trains to run underneath.
This method also prevents the need to make significant changes to historic architecture.
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Network Rail's 'orange army' has completed track-lowering work under the A4 at Box Ashley and through Middle Hill Tunnel on the main line towards London Paddington.
However, while work was underway, it was discovered that some 'catch pits' in the tunnels, part of the railway's drainage system, had deteriorated to a greater extent than expected.
As a result the team has widened the scope of the project to ensure those areas affected are replaced.
Andy Haynes, project director for the west of England, said: "The first week of this complex project has gone well, with all our initial milestones completed on time despite some unexpected challenges.
"This section of track was last renewed in the 1970s, so we knew we could encounter extra work as we went along. We've made sure we give ourselves just enough contingency in our plans to deal with the unknowns which often crop up when working on ageing sections of the railway."
(LM)
