It is shut until 21 October so the work, which is part of the wider £2.8bn scheme to electrify the main line from London to south Wales, can take place.
Three teams of 200 engineers are working 24 hours a day for six weeks.
Their work has included installing a state-of-the-art power rail in the tunnel roof.
Network Rail's Tim Butler described the situation as "unique" with five years' worth of usual work squeezed into six weeks, which includes weeknights and weekends.
"Within the six weeks working we're spending around £10m, so you can understand the scale of the work 24 hours with 200 people around the clock," he added.
Rail replacement buses have been operating between south Wales and Bristol while the tunnel has been closed, while flights have taken passengers from Cardiff to London city airport.
Despite the disruption, Great Western Railway has been providing 32 train services a day between Cardiff and London Paddington.
Work installing the electrified rail from England to Wales has already been finished, while engineers are now putting the finishing touches on the rail in the other direction.
Network Rail also said there was no date yet for when the line would be electrified from Cardiff to Swansea.
In a statement, it said: "Electrification between Cardiff and Swansea is expected to be delivered within control period six (2019-24).
"Specific dates and costs are yet to be confirmed and Network Rail is currently undertaking development work for this phase of the project."
However, the statement said one of the main benefits of electrification would be experienced by passengers in 2017 when the new Intercity Express trains are rolled out.
A six week closure to upgrade the Severn Tunnel for electric trains to use in and out of Wales is costing £10m, Network Rail has said.