A senior executive at one of Wales' most successful companies says he fears the nation will be held back unless there is major rail investment.
David Stevens of insurance firm Admiral says Wales needs "game-changing" trains investment to promote growth.
Mr Stevens, who helped launch a report calling for a south-east Wales "metro" link last week, said people needed to be able to travel to jobs.
He will tell a conference rail projects need not be at the expense of roads.
Admiral, the only Welsh company in the FTSE 100, employs some 3,800 people in Cardiff, Swansea and Newport.
Mr Stevens, the company's chief operating officer, said feedback from staff travelling between offices, and to and from work, was that trains needed to be more frequent and reliable.
"[When there's] long gaps between services, and then they don't necessarily turn up, and the next one isn't for an hour, then that's a big problem," he said.
Mr Stevens added: "I think what Wales needs is a significant investment in its transport infrastructure in general.
"I'm not saying lets transfer investment from road to rail but there's big opportunities for investing in rail services."
He is advocating a "metro" link connecting Cardiff, Newport and the valleys, and better train services generally.
He said electrification in theory could remove some of the barriers to better, more frequent services.
"I don't purport to be a technical expert but investment in lines and stock and the quality of the service will help address these issues and I'm told electrification is probably the best option to achieve this," he said.
'Big missed opportunity'
A report last week from the Cardiff Business Partnership, of which Mr Stevens is director, called for an investment of £2.5bn over 10 years to connect Cardiff, Newport and the valleys through a metro system.
"It [lack of investment in rail] will hold us back. It will be a big missed opportunity," he said.
"There's a lot of research that shows that the most vibrant and fastest-growing parts of the country are cities over half a million people.
"Really you want to be over a million. Cardiff itself is too small but if you could link together the whole region, you're up to 1.4m."
The report said the metro network, combining electrified heavy rail and light rail systems, would be boosted by faster rail links to London and Heathrow - should the UK government give the go-ahead to electrification of the line into Wales.
Last November, the UK government deferred a decision on electrification between Swansea and London Paddington.
A rail conference in Cardiff on Thursday, organised by the Institute of Welsh Affairs think-tank, will explore some of these issues.
It features speakers such as the chair of London's £16bn Crossrail project, Terry Morgan, and senior officials of train operating companies First Great Western and Arriva Trains Wales.
The conference coincides with a visit to south Wales by Labour's UK shadow transport secretary, Maria Eagle, who is calling for the UK government to press ahead with electrification from London to Swansea.