Network Rail wants to drill into a 164-year-old bridge spanning the River Towy to help check its structural condition.
It plans to carry out the work on one of Llandeilo railway bridge's stone abutments in two locations.
The results will determine whether the bridge needs to be reinforced to protect it from a process called scour, which is when fast-flowing water removes sand and gravel from bridge abutments and piers, potentially affecting its structure.
Network Rail's planning application to Carmarthenshire Council said the Llandeilo bridge's external appearance was generally good.
The bridge carries two tracks, although one isn't used, and was built in 1856 for Llannelly Dock and Railway Company.
The lattice girder bridge was granted grade two-listed status in 1991. Parts of it have reinforced steel concrete and sheet piling.
A heritage impact assessment submitted as part of the application said: "The structure has been identified as at risk of the impact of scour, therefore requiring a more detailed physical investigation."
Llandeilo councillor Edward Thomas said he had no objections to essential work being carried out to the bridge, but felt that Llandeilo Town Council and Dyffryn Cennen Community Council should be consulted
The area experienced very heavy rainfall last winter and also during Storm Callum in October 2018.
Network Rail said the planned drilling was a routine inspection.
A spokesman said the bridge was checked for scour above the water line every 18 years and below under the water line every three years.
He added: "In addition, the bridge has a six-year detailed examination and an annual visual examination for condition."
