Transport officials fear rail services between York and Newcastle could be cut - because the East Coast Main Line has become too full.
They are asking Transport for the North, the sub-national body overseen by mayors and council leaders, to fight against possible cuts to services run by Transpennine Express (TPE).
Trains north of York, which stop at Darlington, Durham, Newcastle and Edinburgh, could be cut to just one per hour, down from the usual service of two per hour.
The threat is revealed in a report by officials presented to Transport for the North's rail committee.
It highlights plans to increase services run by London North East Railway, which connect the North East with London.
Because both rail operators use the East Coast Main Line, increasing one service could mean that another needs to be cut, the report says.
It warns: "There is constrained capacity on the East Coast Main Line north of York and currently there is insufficient capacity to accommodate a 7th train on this section of route.
" Thus, trade-offs are being considered by the industry and there is concern from North East Authorities that it may not be possible to run the second TPE train per hour that normally runs on this part of the route."
A consultation on service changes in expected to begin early this year, and Transport for the North will issued a formal response once once that begins.
But the officials suggest the authority should "support the North East in strongly promoting the retention of two trains per hour on TPE’s North East services."
Newcastle North MP Catherine McKinnell, chair of the All-Party Parliamentary Group for the East Coast Main Line, said: "The congested section of track between York and Newcastle is a real bottleneck on the East Coast Main Line which seriously holds back our capacity to add new routes.
"I and cross-party colleagues on the All Party Group on the East Coast Main Line have raised this issue with the Government many, many times and Network Rail have called for the reopening of the Leamside line to provide much-needed relief. Unfortunately we appear to be no further forward."
Transpennine Express referred requests for a comment to Network Rail. Network Rail said it was a matter for the Department for Transport.
