Police are suggesting that rail travellers should text an incident alert - if they see a passenger smoking on a train.
British Transport Police (BTP) is urging travellers to report "crime or incidents" on trains or at stations "discreetly" by text.
An advertising campaign - which has the strapline "let's make a difference" - gives examples of the kind of crimes and incidents travellers might report.
One example on advertisements posted on trains is "passenger smoking".
The head of a group which lobbies on behalf of smokers expressed surprise.
"You can only laugh," said Simon Clark - director of Forest. "I'd be interested to know how many texts they get. I can't remember the last time I saw someone smoking on a train.
"I would have thought you might ask the person to stop or point at the many no smoking signs or tell a member of staff. I'm not really sure there's a need to alert the police.
"I wonder what the police will do when they get the text from one of this army of citizen spies? Will there be a Swat team at Watford Junction ready to follow a trail of smoke."
A BTP spokesman said: "Smoking is a by-law offence. It may make a passenger feel uncomfortable. It may create a safety issue."
He said police might board a train and speak to a smoker if alerted.