More than half a million passengers have used the Borders Railway in its first four months, exceeding original expectations.

The line, which opened in September, is the longest new domestic railway built in the UK for over a century and takes passengers on a 30-mile journey from Edinburgh through Midlothian to Tweedbank in the Scottish Borders.

It re-establishes part of the former Waverley line, which fell victim to the controversial Beeching cuts in 1969, leaving the Borders region without any access to the National Rail network.

The new service has proved popular with both tourists, commuters and the Queen - who spent the day she became Britain's longest-serving monarch travelling on the line - and rail bosses marked the 500,000 journey milestone at Edinburgh Waverley on Thursday.

Initial predictions were for the service to carry 650,000 passengers in its first year.

Transport Minister Derek Mackay said the near £300 million investment in the service has been worth it.

"We knew it would be a success and a wise investment but it's even more popular than we thought," he said.

"I think it's been vindicated and is making a difference. Rail requires huge infrastructure support, and it has that.

"Rail in Scotland is very popular at the moment. This service works because it is reconnecting communities between Edinburgh, Midlothian and the Borders and is creating economic opportunities as well as a commuter link."

ScotRail Alliance managing director Phil Verster said: "We had a sense in September that Borders was going to be popular but no-one expected us to reach 500,000 passengers so quickly."

Sarah Eno and Andy Swales, from Selkirk, got off the train at Waverley to be told they were the passengers who took the service over the 500,000 passenger milestone.

Mr Swales, 67, said: "We were one of the first paying passengers on the train and we've used it a couple of times a month since then. Before we either had to drive or take a not very nice bus journey, so this is luxury compared to that."

Ms Eno, 68, added: "There was initially a big boost and I noticed a lot more people coming to the Borders, but it was a beautiful September and October.

"I think if they put more bike spaces on the trains it would help because the Borders is a terrific cycling area."

The couple believe the success supports the case for extending the line further, and potentially into England.

Mr Swales said: "It deserves to go at least as far as Hawick because that's the biggest town in the area, and travel south to England isn't very easy so it would be great if it went all the way through to Carlisle."

David Spaven, rail consultant and author of ‘Waverley Route: The Life, Death and Rebirth of the Borders Railway’ said transport bosses should not be surprised by the popularity of the service - despite problems with overcrowding and punctuality. 

Mr Spaven said: "The substantial patronage on the Borders Railway is very welcome, but Phil Verster of the ScotRail Alliance is wrong to say that ‘no-one expected us to reach 500,000 passengers so quickly’.

"Rail campaigners certainly did, when they saw the ludicrously pessimistic forecasting undertaken four years ago by Transport Scotland, the Scottish Government agency. The final business case – which concluded that the benefit to cost ratio would be 0.5:1 – envisaged patronage which equated to just over two passengers per train at Tweedbank and a similar number at Galashiels. As campaigners said at the time, even the dismal X95 bus from Galashiels to Edinburgh (journey time 85 minutes, as opposed to 50 minutes on the train now) did much better than that.

"Responding to this supposedly poor business case, Transport Scotland cut back the amount of double track on the railway from 16 miles to 9½ miles, and the knock-on effect is that timetable reliability on the new railway has been far from good. At the last count, only 35.5 per cent of trains were arriving on time at the Tweedbank terminus, and the Borders Railway has been one of the poorest performing ScotRail services overall.

"Remarkably, most people who have tried the train seem to be sticking with it – which probably goes to show just how unsatisfactory their previous transport arrangements were, whether by bus or car."

Mr Verster said expansion plans will be looked at.

"We've had challenges to maintain enough carriages on this service because it has been popular, but our future plans are to add carriages to our overall fleet and we will strengthen services continuously to meet demand," Mr Verster said.

"Every opportunity to extend the railway to Hawick, and even further to Carlisle, must be evaluated and could hold very exciting possibilities for us."