The view had been blocked by foliage
The Box Tunnel's western portal can be seen in all its glory for the first time in decades following a clean-up.
The view of the decorated facade at the entrance to the structure from the viewing platform on the A4 in Bath had been blocked by foliage.
At the time of its completion in 1841, the railway tunnel was the longest in existence and the largest engineering work on the Great Western Railway.
The work on the Brunel-built structure was undertaken by Network Rail.
They removed the plants to allow non-invasive vegetation to grow and for new trees to be planted.
The Grade III-listed structure, which is just under two miles long, runs between Bath and Chippenham.
With Isambard Kingdom Brunel as chief engineer, it was the last-finished engineering work on the Great Western Railway, which ran between Bristol and London.
Jack Hitchcock, Network Rail 's western maintenance director, said the work was to make the "piece of history" more accessible.
The work also pays tribute to the 204th anniversary of the engineer's birth on 9 April 1806 and the 175th anniversary of the Great Western Railway.
He added: "Box Tunnel is one of Brunel's masterpieces on Great Western and despite its age, still plays an important part in today's modern railway."