Organisers of a Thomas the Tank engine 60th anniversary party say they have been "overwhelmed" by demand.
The 15,000 tickets for a party featuring the little blue engine at Hatfield House, Herts, on Saturday sold out within days of it being announced.
Tickets have been traded for more than 20 times their original price of £10.
The anniversary celebrates the first publication of the Rev W Awdry's The Three Railway Engines in which Thomas later became the star.
They were originally just made up tales told to his son Christopher when he was ill in bed with measles in the Wiltshire village of Box.
When Christopher told his father off for not being consistent with the details of the tales, the clergyman wrote them on scraps of paper.
His wife Margaret then persuaded him to send them in to a publisher and they became a worldwide phenomenon.
Thomas the Tank engine TV shows are now shown in more than 130 countries.
More than 80 million Thomas books have been sold since 1945.
A spokesman for organisers Hit Entertainment said: "We can understand people's disappointment but we hope people will not just turn up. It is completely sold out.
"There will be other Thomas events to celebrate throughout the summer and we hope people can look out for those."