The effort to spruce up the Gresley LNER class A4 No. 4498, also known as A4 60007 Sir Nigel Gresley, has taken six years to complete.
There will be a short celebration to mark the engine's return to the tracks at the Kidderminster station at midday on April 21 .
Nigel Wilson, chairman of The Sir Nigel Gresley Locomotive Trust, which owns No. 4498, said: ”The event marks the return of our popular A4 class locomotive following the tremendous efforts of our essentially volunteer engineering team.
"This overhaul is probably the most thorough the locomotive has ever had. Thanks to all involved and the results of the hard work are now for everybody to enjoy. We’re also very grateful to the Severn Valley Railway for hosting us for the all-important running in tests.”
The loco is named after the renowned railway engineer Sir Herbert Nigel Gresley CBE, who was the chief mechanical engineer of the London and North Eastern Railway. He died, aged 64, in 1941.
Some of his belongings are on display at Severn Valley's Highley station.
Sir Nigel Gresley loco, brother of world speed record holder Mallard, is now the only operational A4 locomotive. It is is owned by a charitable trust which aims to keep it steaming.
It will operate at the gala from Thursday April 21 to Sunday April 24 before moving on to the Locomotive Services Group depot at Crewe where it will be prepared for main line leisure trips from May onwards.
The overhaul which was launched in 2015 was completed last month prior to a period of running in at Severn Valley. The loco is currently in a temporary livery of LNER black which it carried in the 1940s.
Later this year it will be repainted in British Railways express passenger blue and returned to its other running number of 60007.
The trust purchased the locomotive from British Railways in 1966 and has been maintaining and operating it ever since.
Severn Valley Railway’s managing director Helen Smith said: “It’s been a pleasure to host this iconic locomotive at the railway for its running in tests in recent weeks.
"We’re delighted that it’s staying to make an exclusive guest appearance at our Spring Steam Gala, where we know it will prove tremendously popular. I’d also recommend visitors take the opportunity to visit The Engine House at Highley, where Sir Nigel Gresley’s famous top hat is on display.”