Iarnród Éireann has warned that it may be forced to introduce pre-book only trains in response to overcrowding on the rail network.

Speaking in an RTÉ Prime Time report airing tonight at 9.35pm, CEO of Iarnród Éireann, Jim Meade said that crowding on trains could get worse before it gets better as additional carriages will not arrive until late 2021 at the earliest.

Passenger numbers are at record levels with 48 million journeys undertaken in 2018 and the volume of rail travel is expected to be higher again this year.

Iarnród Éireann has a number of initiatives in place encouraging passengers to avoid the busiest periods, however, a more drastic measure is also being considered.

Pre-booked trains, for which tickets can only be bought in advance, are usually only operated on busy event days such as All-Ireland finals.

However, Iarnród Éireann is considering introducing the restrictions on regular routes which means some peak-time trains could be sold out in advance.

"That would put a restriction on people's ability to travel because currently they can pre-book or they can walk up and go but certainly in order to manage capacity it may be something we have to look at in the long term," Jim Meade explained.

"With the current growth patterns continuing there is a risk that it (crowding) will get worse before it gets better. The issue for us is how we manage that capacity," he added.

Forty-one new carriages are in the process of being ordered, however the lead time to deliver such stock is two years.

Trade unions and rail users say that crowding on trains has reached a chronic level.

"The intercity trains are horribly overcrowded. People have to climb over each other to effectively get out of the carriage," said Mark Gleeson of the campaign group, Rail Users Ireland.

"It is a common enough occurrence across the network that the train is shorter than expected. Because there is no spare if something breaks down a train is reduced in capacity."

Dermot O'Leary of the National Bus and Rail Union said the pressure on services should have been anticipated, and new carriages ordered earlier.

"The dogs on the street knew that the population was exploding and for the population to explode they would need transport provision.

"What really frustrates people in the industry like myself, is that we are not talking about building new railway lines here we are talking about using the existing footprint. And just putting extra capacity on."