The unexpected cargo disrupted passengers on the Virgin Trains East Coast service heading to the Scottish capital on Sunday afternoon.
Journalist Harry Horton tweeted about the surprise arachnid and said he first noticed something strange about his trip when passengers in the seats before him started standing up and retreating.
He said: "There was something going on at the end of the carriage and I couldn't quite see what it was.
"A lot of the passengers were up on their feet. All of a sudden a couple came down to my end (and) they said there's a scorpion on the loose."
A guard appeared shortly after and the scorpion was returned to the ice cream container it had been stowed in, Mr Horton said.
But the travellers still had to endure an eight-minute delay at Peterborough after the affected carriage containing about 20 people was sealed off and checked, he said.
He said the scorpion appeared to belong to a woman who was in a hurry to continue her journey - so much so, she left the creature behind.
"We weren't quite clear why she had (it)," he said.
He added: "The police basically said to her, you're either getting off here with your scorpion or we're taking your scorpion from you."
"In the end, she decided to give up the scorpion and give it to the police."
A British Transport Police spokeswoman said: "We were called at 2.56pm to a Virgin train from King's Cross to Edinburgh and we met the train at Peterborough.
"A scorpion had escaped from a passenger's bag, but it had been recaptured quickly and put in an ice cream box.
"We have taken the scorpion to an exotic pet rescue in Peterborough."
