Simon Hart MP has discovered that the busiest train line in Pembrokeshire has the worst service.
New rail user figures have revealed that 28,000 more passengers per year use the Pembroke Dock line than the Milford Haven line.
However the northern line has 11 services a day compared to nine a day in the south of the county.
"I have had a number of constituents point out this crazy situation and they undoubtedly have a point," said Mr Hart.
"The services to the south of the county have been gradually eroded with trains stopping at Carmarthen rather than continuing to Pembroke Dock as they used to, or single-carriage trains when we were promised two carriages.
"And yet the southern line has been the busier of the two for at least the last three years. We are getting a second-class service on the busiest line."
Mr Hart has now written to the Welsh Transport Minister Ken Skates to lobby for improved services in the south of the county.
Arriva Trains Wales will not operate the trains on both lines beyond October this year when its franchise comes to an end. The franchise process to find a new operator is now being overseen by the Welsh Assembly Government following the devolution of further powers on January 1.
"I have passed on my constituent's concerns and examples to Mr Skates and am pressing for the new operator to improve services to the south of the county," added Mr Hart.
Simon Hart MP
House of Commons
15, St John St
or 0207 219 3000