Transit police have detained people riding outside of trains twice so far in December, and the authority is pleading with the public to avoid the risky and illegal behaviour.

"If you just think about it for a minute you realize that if you were to slip and fall, and slipping off the coupling would be so easy to do, you will almost certainly fall under the train and you're risking incredibly serious injury, if not death," said Anne Drennan, who speaks for the Metro Vancouver Transit Police.

Riding on the outside of the train is an extremely dangerous practice which could lead to serious injury or death - Metro Vancouver Transit Police

On Dec. 9, a 20-year-old man was ticketed with fines worth $115 and $173 after he was found riding on the coupler head shortly after midnight on a SkyTrain heading eastbound towards Royal Oak Station.

He jumped on the platform there, but was later intercepted at 22nd Street Station by officers.

Transit Police say the man had been drinking and told them that he was celebrating his 20th birthday.

After the Chilliwack resident was fined and, "warned as to the extreme danger of his actions," he was released.

"Riding on the outside of the train is an extremely dangerous practice which could lead to serious injury or death," wrote Metro Vancouver Transit Police in a release on Saturday.

"It also causes an inconvenience to other passengers as it can lead to a guideway intrusion alarm, which causes delays."

Transit Police also detained a pair of teen boys, aged 13 and 14, who were found riding the outside of an incoming train to Stadium Station just after 1 a.m. PT on Dec. 4.

The teens, one from Vancouver and the other from Coquitlam, were turned over to their parents without being ticketed because of their ages.

Drennan says that officers are searching the internet to see if there is something out there encouraging people to engage in the risky behaviour.

"We're looking online to see if there might be sites that are suggesting that this is a cool thing to do given that we've had three of them doing this in the last few days," she said, adding that the occurrence is otherwise fairly rare.

The authority is asking anyone who sees someone riding on the outside of a train to text 87 77 77 or call 604-515-8300.