The government has admitted the electrification of the Great Western main line faces "challenges" amid peers' questions over delays.Last week, ministers indicated the timetable for electrifying the line to Bristol Parkway by 2016 and Swansea by 2018 may be pushed back.
Lib Dem Baroness Randerson asked if the government had committed enough money.
| HST's will have to stay in service longer than planned |
if the scheme is delayed
"The government is committed to delivering electrification from Paddington to Swansea," the Conservative peer said.
"There are challenges which have arisen from electrification and construction and planning consents have taken longer than expected."
He said Transport Secretary Patrick McLoughlin had asked Network Rail chairman Sir Peter Hendy to report back on "issues of deliverability and affordability" by the autumn.
In June the government said it would delay or cut back a number of modernisation projects planned for Network Rail but said the Great Western line was a top priority.
Network Rail's £50m programme aims to cut journey times on the route between London Paddington to Swansea.
But on Monday, Mr McLoughlin told the Commons Transport Committee electrification was "providing some very big engineering problems".
Asked whether the line to Oxford, Chippenham and Bristol Parkway would be electrified by December 2016, as planned, he said: "I can't give you a yes or no answer."
Earlier this week, Welsh Secretary Stephen Crabb could not confirm the electrification of the line to Swansea would be done by 2018, as planned.