Workmen found the brightly-coloured advertisement, which dates from 1962, while removing cladding at Bidston station.
It details a show at the Tower Ballroom in New Brighton headlined by Little Richard.
The poster for the October 12 gig also names The Beatles, who are said to be playing in a “sensational line-up of top British rock groups”.
Merseyrail bosses are now trying to decide what to do with the poster.
Paul Collins, Merseyrail projects engineer, said: “The contractors on site were ripping some boards off when they found the poster.
“They knew straight away that it was something important and needed to be saved.
“It looks like there are three other posters underneath and maybe there’s a company out there who can separate them.
“We have found some real gems in the past at train stations.”
The Beatles played at the Tower Ballroom 27 times.
The venue had an illustrious musical history, with stars including The Rolling Stones and Jerry Lee Lewis playing there before it was destroyed by fire in 1969.
Geoff Nugent of Merseybeat band The Undertakers, played on the same bill as the Beatles for the gig advertised on the poster.
He said: “Just to be part of a big show like that was great – we loved it because there were few big venues in those days.
“We would mingle with the crowd, but the Beatles were kept in their dressing room until their performance – they were an enigma.”
Doug Darroch, curator of the Merseybeat museum at Fort Perch Rock in New Brighton, said he hoped the poster would stay in Wirral.
He said: “It is significant because it shows how Brian Epstein was trying to push The Beatles in a big way.
“Part of that strategy was bringing in big stars like Little Richard and putting The Beatles on as the second biggest act.
“A finding like this is pure musical history.”
By Lorna Hughes