MP Amber Rudd joined representatives from local rail groups when they met transport operator GoVia at Rye Town Hall last Friday.
Among the issues raised for the Rye line were the need for more carriages to alleviate overcrowding and a better Sunday service for the smaller stations.
The meeting is one of a series held with firms bidding to run local rail services from next year under the Thameslink Southern and Great Northern franchise.
Meetings have previously taken place with rival bidders First Group and Abellio, while talks will take place with bus and rail operator Stagecoach later this month.
East Sussex Rail Alliance spokesman Ray Chapman said: “We all outlined the importance of a strategic vision for rail services along the south coast corridor and beyond, and the need to upgrade services on the Marshlink line between Hastings and Ashford in particular.
“This is going to be a very large train operation covering from Ashford in the east to Southampton in the west, and as far north as Kings Lynn and Milton Keynes.
“There will be some big changes under this franchise which will set the pattern for local rail services for many years to come. It is important for us to get in on the ‘ground floor’ to influence how future rail services are provided.
“The Department for Transport’s Invitation to tender has laid out the groundwork and taken on board many of our suggestions. We must now help bidders fill in some of the detail.”
Stuart Harland, chairman of the Marshlink Action Group, said: ““We are all very pleased to see that over the last few years the Marshlink has transformed from a quiet rural branch line into part of an increasingly important inter-regional route with national and international connections.
“Whereas 20 years ago there were genuine concerns about the future of the line, it is now increasingly a victim of its own success.
“We told the GoVia team that there is an urgent need for additional carriages to alleviate overcrowding.
“At the same time, it is important that the rural communities that depend on the Marshlink are not forgotten, and that services are maintained to the smaller stations and reinstated on Sundays.
“ But there is no use publishing a timetable if it doesn’t operate: So all the groups present expressed deep concern about the deteriorating reliability of the rolling stock in recent months and asked the Govia representatives to put these concerns to their colleagues at Southern Railway, the current operator.”
Amber Rudd said: “I believe that the aspirations of local commuters and rail user groups have been heard by the Department for Transport and the rail industry. I am increasingly optimistic that this hard work will pay-off in terms of improved services and extra investment in local lines in the next few years.”