The Government has been urged to encourage more competition on the rail network to boost passenger satisfaction and reduce fares, according to a new report.
The lack of long distance routes served by multiple operators is an "underlying problem for the UK's railways", according to the economic bulletin published by the Centre for Policy Studies think tank.
The study's authors said, apart from in a few cases, competition "effectively ceases" after franchises are awarded.
Grand Central and First Hull Trains are the only two long distance open access operators currently running trains.
The report noted studies which showed there is a high level of passenger satisfaction on these services and fares for the routes are more competitive than elsewhere.
It described the regulations for open access proposals as "restrictive", and said the services are mainly approved to "fill a gap" rather than provide competition to franchise holders.
Concerns that changing this policy could pose a risk to the revenue streams of franchisees - which would in turn reduce payments made to the Government - were also acknowledged.
In March the competition watchdog called for rail franchising to be overhauled and possibly replaced by multiple operators running services on the same lines.
The Competition and Markets Authority (CMA) said this could lead to lower fares, greater incentives for rail firms to improve services and more effective use of network capacity.
The report distributed by the Centre for Policy Studies said the Government "has yet to formally declare its support" for the principle of open access.
It added: "The CMA has recently called for greater on-track competition on long distance routes. It is now time for the Government to follow suit."