CIE and the RPA are fighting for control of a disused railway line running from the former Broadstone railway station in Phibsborough to the Maynooth line.

The line is owned by CIE but, under the government's Transport 21 strategy, it is to be transferred to the RPA so it can build the Luas line.

However, Irish Rail, a subsidiary of CIE, has now put forward a rival scheme for the land, which would see the reopening of the line.

The RPA and the Department of Transport are determined to proceed with the Luas scheme however and have refused to rule out serving a compulsory purchase order on CIE for the line.

An RPA spokesman said that the agency did not need CIE's approval to build the line. "The RPA is not part of the CIE group and applies to An Bord Pleanala for powers to construct, operate and maintain lines, " he said.

CIE has denied that Irish Rail's proposal is designed to block the Luas line proposal. "It is about meeting future demand and ensuring we have a network that has the capacity and flexibility to ensure we can do that, " said a spokesman.

"We envisage that Broadstone would be a terminus for Navan commuter services. It would also facilitate pre-Interconnector service expansion for the Maynooth line."

He said that Irish Rail will submit a business case for the project to the department in the near future. He said that the line would cost between 30 million and 40 million to reopen and services could begin as early as 2010 if the government approved its plan.

However, a spokeswoman for the department said that it would not approve CIE's scheme. "The Luas line has already been decided for the project and that will not be changed, " she said.

She declined to answer questions about whether the dispute revealed a lack of coordination between the transport agencies under the department's remit.

A spokesman for national railway user group, Platform 11, which represents Luas and rail passengers, said the dispute was a "turf war" in which passengers would lose out.

"In this case, it's clear that the Luas link delivers more than a railway would. It could have more stops and bring people right into the centre of town, " he said.