Overcrowding on morning rail services will get worse before it gets better, Irish Rail has admitted.
Chief executive Jim Meade said the lack of capacity would not begin to be resolved until 41 new carriages arrived in 2021, with further improvements when 100 new hybrid vehicles entered service in late 2023.
Passenger numbers hit a record high last year of 48 million, but the existing 629-strong fleet cannot accommodate existing numbers in the morning peak.
This has led to widespread criticism about passengers being forced to stand in the morning and evening rush hours, despite paying expensive fares.
"To be fair to our customers, we have to say it may get a little worse in the morning peak, but from 2021 and beyond, we will see improvements because we'll see capacity back on the system," Mr Meade said.
"We need to increase new fleet because we're headed for capacity.
"We have to recognise that the vast majority of people will want to travel in the morning peak.
"We have to say 'you won't be getting a seat [until the new fleet arrives]'."
The first of the new fleet - 41 carriages - will be ordered in the coming months, at a cost of around €2.2m each.
These carriages will be used to extend commuter and inter-city trains, and up to 100 could be procured over time.
These will have the capability to be converted to hybrid usage - meaning they can run on diesel or battery power.
"That gives us capacity," Mr Meade said.
"Three-carriage train-sets may be extended to four or five.
"We're going to move them to where they're needed, and around 80pc or 90pc will be for the Greater Dublin area.
"The longer term is complete new electric trains, which will also be hybrid. They can run under wires or under traction.
"We're considering a framework order to allow us to purchase 600 over a 10-year timeframe. We would hope to place the first of that order, around 100, for 2019, with late 2023 for delivery."
The additional fleet would allow a 10-minute service from Hazelhatch and Maynooth in Kildare to be provided, similar to that on the Dart.
"We've seen consistent growth," Mr Meade added.
"Last year will have seen just north of 5pc, which follows growth in excess of 6pc in 2017.
"We saw the recovery in the Dublin area first, but 2018 is when we saw the big jump across the rest of the country.
"The Government investment is about increasing the frequency of travel to Maynooth and Hazelhatch, and expanding to a 10-minute service.
"But the nature of heavy rail is it takes time to deliver."
Mr Meade also said it was unlikely the company would go ahead with plans to lease trains as a short-term solution to tackling capacity.
This is because carriages required to be modified for use on the Irish rail network would need to be approved by the Commission for Railway Regulation, which would involve additional driver training and could end up being more expensive than buying new.
On the issue of anti-social behaviour on trains and the possible need for a dedicated transport police, Mr Meade told the Herald: "It's a real issue.
"We have developed a very good relationship with An Garda Siochana and we get a very good response.
"That said, it's something I don't believe will go away.
"We've put a lot of effort into security at our stations but I don't think a transport police is required. I don't believe we're big enough for that."