Engineers will complete a structural survey of the railway bridge at near Arrochar and Tarbet station from 20:00 on Thursday 13 June to 06:00 on Friday 14 June.
Signage will be in place ahead of the road closure with a diversion route in place via the A82, A85, A819 and vice versa.
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A Network Rail spokesperson said: "The work will provide vital information on the condition of the railway bridge as part of our efforts to protect the long-term reliability of the line.
"Working on the railway at night offers the safest environment for our teams and causes the least disruption to staff, passengers, freight trains and motorists.
"We would like to thank those affected for their patience and understanding and offer assurances that every effort has been made to reduce the impact of our activities as much as possible."
