Commuters in the Midlands and London face at least a week of disruption after a rail tunnel collapsed.
It is thought that tons of rubble are on the railway line
Tonnes of earth dropped on to the track at Gerrards Cross, Buckinghamshire, near to where a new Tesco store is being built.
No trains were in the tunnel at the time on Thursday evening, although one was stationary nearby.
An hourly service is now running between Birmingham Snow Hill and London Marylebone via Aylesbury.
Rail replacement buses are operating throughout the day between Beaconsfield and Amersham, High Wycombe and Maidenhead and Denham and West Ruislip.
Chiltern Railway tickets will be accepted by Virgin Trains, First Great Western Link, Central Trains, Silverlink Trains and London Underground.
Robin Gisby, head of operations for Network Rail, said he thought it would take at least a week to restore rail services on the route.
"The tunnel has collapsed. We've now got to assess today why that happened. Experts are in there right now and they will work through the tunnel and decide what was the cause of the delay," he said.
"Then we're going to make it safe, then we're going to work out how we're going to clear up the considerable problem we've got.
"There's a couple of thousand tonnes of material in there that we've got to move.
"We've got to get the concrete structure, that has collapsed, out. We've probably got to do all of that by road.
"Although we brought the material in by rail, we don't think we can get it out by rail, but we won't assess that until later on today and through the night.
"I think that's going to be at least a week's work, possibly longer, before we can restore rail services on this route."
Chiltern Railways said safety inspectors were on site investigating the cause.
"Passengers are being advised to travel via alternative routes and buses have also been ordered. We are currently putting arrangements in place to minimise disruption."
Network Rail said there were no reports of injuries after about 20 metres of tunnel collapsed at 1915 BST.
Eyewitness Brett Jackson told BBC News 24: "I was parking my car when I heard what sounded like a clap of thunder - I thought it was an earthquake.
"I saw the tunnel falling on to the rail track."
A Tesco spokeswoman apologised for the disruption to services.
She said: "We have launched an urgent investigation with our engineer contractors and will work with the authorities to understand what happened and get rail services back to normal as soon as possible."
The store was in the "very early" stages of construction and was being built above the line.