Smoking was allowed on Underground trains until 9th July 1984, latterly in two cars per train (normally the trailer immediately next to each driving car).
On the C stock, trailers were smoking if marshalled next to the driving cars at either end, but otherwise they were non-smoking. Because of this, the intermediate trailer cars had slots at the bottom of each window into which "no smoking" roundels were inserted when required, while the driving cars were always non-smoking so they had "no smoking" roundels permanently affixed near the tops of all windows.
I was 9 in 1984, and I can remember seeing posters at Underground stations announcing the smoking ban on trains. For the first few weeks after that date, the erstwhile smoking cars had paper "no smoking" roundels on the windows, until these were replaced by permanent "no smoking" signs in due course.
I think smoking was banned on the Underground in three stages, firstly on trains, then in below-ground stations (probably because of fires such as that at King's Cross in 1987) and lastly at surface stations.
Of course, in the days when trains had smoking cars, it was tough luck if you were a non-smoker and got on a train at the last minute then found after the doors had closed that you were in a smoking car (or, conversely, if you were a very dependent smoker and found that you'd got in a non-smoking car by mistake).