First Great Western services between London Paddington and south Wales have resumed after a 2,000-tonne landslip, but disruptions are continuing.
The slippage, near Chipping Sodbury on Thursday, closed the line between Swindon and Bristol Parkway stations.
The train operator said the line had reopened and full peak services were expected to run on Friday.
Passengers have been warned that all journeys would be up to 30 minutes longer due to temporary signalling.
Off peak, only an hourly service will run in both directions between London Paddington and Swansea.
A bus service continues to operate between Bristol Parkway and Patchway stations.
Network Rail said a 20mph speed limit put into place between Bristol Parkway and Swindon on Thursday was being increased to 40mph, which should help to reduce the delays.
Route director Dave Ward said: "Engineers have been on site through the night with a huge logistic team behind them, involving the whole industry.
"We managed to clear the 2,000 tonnes of rock using special trains to get it away.
"Where the earth has fallen, it has severely damaged trackside equipment, including power to the signalling itself."
The replacement signals will be tested over the weekend, when the line was due to be closed anyway for engineering works.
'Act of God'
Some passengers had praise for the way the incident had been handled.
One told the BBC: "It's a bit of a pain, because it's going to take longer, but it's only half an hour, so it could be worse. They've done quite well to get things organised."
Another said: "It's a bit annoying, but you can't stop a landslide I guess. The website information is good. It was an act of God, I suppose."
The landslip is being blamed on heavy rainfall which saturated the embankment.
It is another blow for First Great Western, which has been severely criticised over delays, cancellations and over-crowding.
There are hopes that services will be back to normal by next Monday, when a proposed fare strike by disgruntled passengers is expected to be staged.