The organisation that represents train operators and Network Rail said there were 1.7 billion journeys made by rail in the UK between 4 January 2015 and 9 January 2016.
That was 62 million more than in the previous year.
The RDG said that cheaper tickets sold in advance and off-peak tickets had grown particularly strongly.
But even so, sales in cash terms rose by 6% to Â£9.4bn.
Regulated train fares were allowed to rise by 2.5% in 2015.
You might have heard people say we have the most expensive train fares in the Europe - and it's true. But the UK also has some of the cheapest, if you can book in advance and travel at quieter times.
These cheap deals are one reason why our trains are more popular than ever.
But attracting millions of extra passengers has a downside. Our Victorian network is squealing under the strain, with reliability, overcrowding and punctuality getting worse.
And they simply cannot upgrade the network quickly enough, because the lines are in near-constant use. Engineers only have a three to four-hour gap in the middle of the night to carry out work. There's only so much you can do, safely, in that time.
The only other option is to blitz the work by shutting whole lines for weeks on end, but passengers simply won't stand for that.
So more passengers means more fare money coming in. But so far, improvements haven't kept pace with this rapid growth.
Sales of super off-peak tickets and anytime fares were both up 10%, while off-peak and advance tickets rose by 5%.
No figures were available for which ticket types suffered a slide in sales.
The biggest growth in sales came in London and the South East, where 4.3% more tickets were sold, while there was a 3.6% rise for seats on long-distance routes.
The number of UK train journeys made last year rose 3.8%, according to the Rail Delivery Group (RDG).