Reform Scotland highlighted the figure as it called for the organisation - responsible for maintaining rail infrastructure - to be devolved to Scotland.
The move would offer a “clear line of accountability when something goes wrong”, it said.
The recommendation is contained in the think-tank’s new report on modernising Scotland’s railways.
It comes as ScotRail faces ongoing criticism over delays to its services, with calls for franchise holder Abellio to be stripped of the contract.
Some critics want the franchise to be run by a public-sector operator in the future.
Reform Scotland’s report found that, over the last year, 54 per cent of ScotRail’s delays of more than three three minutes were a result of faults attributed to Network Rail, which is already a public body.
Network Rail is funded by Transport Scotland but is ultimately accountable to Westminster, not Holyrood.
One of the report’s authors, Tom Harris, said: “The current debate over whether or not to nationalise ScotRail misses the point. 54% of delays are the fault of Network Rail.
“Nationalising ScotRail won’t make the trains run on time and it is self-defeating for any politician to imply otherwise.
“Instead, we need fundamental change to the governance of Network Rail.
“The Scottish Government is responsible for the strategic direction and funding of the Scottish rail network, but this responsibility cannot be properly exercised while Network Rail remains answerable to the UK Government.
“Reform Scotland believes that Network Rail in Scotland should be fully accountable to the Scottish Government, and that means it must be devolved.”
Other issues identified by the report include unfavourable journey times in Scotland compared with those of a similar distance in England - Edinburgh to Aberdeen and London to Birmingham are roughly the same distance, but the former takes well over two hours and the latter less than 90 minutes.
While it welcomed investment in the railways, it said upgrading lines while trying to use them can cause additional problems and investments such as the Borders Railway had been “short-sighted”.
The new line from Edinburgh to Tweedbank is not electrified and is single-track.
It has called for the Scottish Government to “future-proof” all new rail investment by ensuring it is double-track and electrified where possible.
The report found that, under current proposals, over the next thirty years people will be able to travel by train from the central belt to London faster than from the central belt to Inverness, despite the former being around 400 miles while the latter is only 160 miles.
It says the government should also create a Scottish rail infrastructure commission to look into faster rail links for the future.
Mr Harris added: “Ongoing improvements are vital and the Scottish Government should be congratulated for investing in this area, but we need to think about the bigger picture and create a bold strategy for the future.
“Our rail infrastructure is deficient. We should not tolerate a situation where a resident of Glasgow can get to London faster than she can get to Inverness because Scotland’s cities are too poorly connected.”