Damage estimated at £20,000 was caused to the railway at Moss Bank Park in Bolton, a spokeswoman said.
Volunteers at the attraction believe it was a targeted theft of the valuable aluminium metal tracks.
Greater Manchester Police said enquiries were ongoing and asked anyone with information to get in touch.
"It is soul destroying," said Tracey Stirling, of Moss Bank Park Model Engineering Society.
"They have taken the majority of the track.
Only 20ft (6m) of track remains of the one mile (1.6km) railway, a spokeswoman said
"There's always been a railway at Moss Bank Park, but nobody has ever destroyed the rails.
"We can't just patch it up - we will have to start afresh."
The railway at the park, which had reopened two months ago after two years of restoration by local volunteers during the coronavirus pandemic, is about one mile (1.6km) long.
Mrs Stirling said only about 20ft (6m) of track remained and damage was also caused to a storage building.
The miniature railway recently reopened after being restored by volunteers during the coronavirus pandemic
A crow bar left at the site had been passed on to police, she confirmed.
"We have had visits from people who remember coming with their grandparents. It's a real focal point for the community and very special to local people," Mrs Stirling said.
"So many people have worked so hard to restore the railway. This is utterly heart breaking, but we will pick ourselves up and start again."