The National Bus and Rail Union (NBRU) has denied allegations from Irish Rail that DART drivers are involved in unofficial industrial action.

The company is threatening legal action against the NBRU and SIPTU, after DART drivers refused to train in new staff.

The NBRU says drivers have the right to refuse to train new recruits.

But the company claims drivers are in breach of contract, and says they are involved in "unlawful activity".

Irish Rail and the drivers unions are currently in negotiations over pay.

The company  had to postpone plans to increase the frequency of DART services in recent weeks.

Barry Kenny is corporate communications officer with Irish Rail.

He told Newstalk Breakfast they have been trying to implement in-cab familiarisation - meaning drivers would sit and observe trainee drivers.

While Dermot O'Leary of the NBRU says management have refused to meet with them.