There are reports that the rail line between south Wales and London is to be electrified, with an announcement possible as early as Thursday.
Last week a report by Welsh MPs said electrification between Swansea and London should be a top priority.
Prime Minister Gordon Brown confirmed he would announce a "major investment" in electrification on two lines.
That follows newspaper claims that the UK government is "finalising plans" to electrify the Great Western line.
The report said this would be part of a drive to reduce carbon dioxide emissions from transport and would involve electrifying the route from London Paddington to south Wales, including Oxford, Reading and Bristol.
The Welsh affairs select committee said it would also lead to significantly better and faster services for passengers.
The Department for Transport (DfT) responded to the committee's report by saying it had been "working closely" with the Welsh Assembly Government "to examine the detailed case for electrification of the Great Western main line between London and Swansea".
The Guardian claimed the electrification of the line would be staged over the 10 years to 2020 to minimise the delays and disruption it would bring.
'Wet and damp'
But rail expert Christian Wolmar played down the idea of an electrified rail line reaching south Wales any time soon.
He told BBC Radio Wales: "There has been talk about this for years and years, but hold your horses, it's not going to happen instantly.
"Probably this will to be carried out in stages. There would be initial electrification to places like Newbury, effectively London suburban trains.
"Then they might do it to Bristol and eventually through to Cardiff but possibly Swansea, then maybe right to the end of the line, who knows?
"But that's really way ahead and there is a big obstacle, of course, between London and Wales, and that's the Severn Tunnel.
"It's pretty wet and damp in there and there might not be enough room to electrify it without digging out the tunnel somewhat, so it will be a big barrier.
"I'm 60 soon and I have my doubts whether I will ever travel on an electrified train all the way through to Cardiff. But I might travel on the [electrified] London suburban network."
A DfT spokesperson said: "As we made clear in the document 'Building Britain's Future, as part of our continued shift towards low carbon modes of transport we plan shortly to set out plans for a major programme of rail electrifications."